Isometric engine problems

Hello. Recently, I started developing a strategy game(in C# and XNA) similar to games like Rise of Nations and Age of Empires(and adding some RPG elements). I've got the games' functions and ideas written down, but I digress.
There is currently ~~one problem and~~ 2 things that I can't figure out.

1. The Problem (SOLVED!)
I've created a basic map drawing method and soon after, the camera controls. The problem
is that at the speed of 1p/10ms (pixel/milliseconds), while scrolling to the right, left and bottom, everything's fine, but when I scroll upwards, lines(from the black background) appear on the ground. The size of the line depends on the speed of scrolling(The interval is always 10ms). Scrolling to the sides and bottom also "produce" lines, but it's only if the
scrolling speed is 2p/10ms or higher.

Game1.cs:

Code:

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.GamerServices;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Media;
using System.Threading;

namespace WarOfGlory
{
   public class Game1 : Microsoft.Xna.Framework.Game
   {
   GraphicsDeviceManager graphics;
   SpriteBatch spriteBatch;

   Texture2D mouse;
   MouseState ms;

   Tile thetile;
   Tile tile = new Tile();

   Texture2D tileTexture;

   public int cameraX = 400;
   public int cameraY = 20;
   public int scrollSpd = 2;
   Thread scrolling;
   Rectangle Left;
   Rectangle Right;
   Rectangle Top;
   Rectangle Bottom;
   MouseCursor mc = new MouseCursor();

   SpriteFont simple;

   public byte[,] testmap;
   Map map = new Map();

   public Game1()
   {
   graphics = new GraphicsDeviceManager(this);
   Content.RootDirectory = "Content";

   }

   protected override void Initialize()
   {

   testmap = map.GenMap();



   Left = new Rectangle(0, 0, 20, graphics.PreferredBackBufferHeight);
   Right = new Rectangle(graphics.PreferredBackBufferWidth - 20, 0, 20, graphics.PreferredBackBufferHeight);
   Top = new Rectangle(0, 0, graphics.PreferredBackBufferWidth, 20);
   Bottom = new Rectangle(0, graphics.PreferredBackBufferHeight - 20, graphics.PreferredBackBufferWidth, 20);

   scrolling = new Thread(new ThreadStart(ScrollMap));
   scrolling.Start();

   base.Initialize();
   }

   protected override void LoadContent()
   {
   // Create a new SpriteBatch, which can be used to draw textures.
   spriteBatch = new SpriteBatch(GraphicsDevice);

   mouse = Content.Load<Texture2D>("Mouse");
   simple = Content.Load<SpriteFont>("Simple");

   // TODO: use this.Content to load your game content here
   }

   protected override void UnloadContent()
   {
   // TODO: Unload any non ContentManager content here
   scrolling.Abort();//put here because of the proccess not turning off, after the red X is pressed
   }
   protected override void Update(GameTime gameTime)
   {
   mc.newState = Mouse.GetState();//get the state of the mouse


   mc.oldState = mc.newState;//sets the old state of the mouse
   base.Update(gameTime);
   }
   protected override void Draw(GameTime gameTime)
   {
   GraphicsDevice.Clear(Color.Black);

   spriteBatch.Begin();
   DrawMap(testmap, spriteBatch);
   DrawMouse(spriteBatch);
   DrawCameraCoords();
   spriteBatch.End();

   // TODO: Add your drawing code here

   base.Draw(gameTime);
   }

   private void DrawMouse(SpriteBatch sb)
   {
   ms = Mouse.GetState();
   sb.Draw(mouse, new Vector2(ms.X, ms.Y), Color.Blue);
   }

   public void DrawMap(byte[,] map, SpriteBatch sb)
   {
   byte lastTile = 0;
   Rectangle source = new Rectangle();
   for (int x = 0; x < map.GetLength(0); x++)
   {
   for (int y = 0; y < map.GetLength(1); y++)
   {
   if(map[x,y] != lastTile)
   {
   thetile = tile.GetTileById(map[x, y]);//gets the tile info
   tileTexture = Content.Load<Texture2D>(thetile.texture);//loads the tile's texture
   int tilesInRow = tileTexture.Width / thetile.tileSizeX;//gets the tiles in row
   //gets the tile into a rectagle for drawing
   source = new Rectangle(thetile.tileSizeX * (map[x, y] - 1) - ((thetile.location / tilesInRow) * tileTexture.Width),
   (thetile.location / tilesInRow) * thetile.tileSizeY, thetile.tileSizeX, thetile.tileSizeY);
   }

   //Problem's sowhere in here
   //tileSizeX(Width) = 34
   //tileSizeY(Height) = 17
   //Camera X/Y is the camera position

   Vector2 drawPos = new Vector2((x * (tile.tileSizeX / 2 ) - y * (tile.tileSizeX / 2 )) + cameraX,
   (x * ((tile.tileSizeY-1) / 2) + y * ((tile.tileSizeY) / 2)) + cameraY);
   sb.Draw(tileTexture, drawPos, source, Color.White);
   }
   }
   }

   private void ScrollMap()
   {


   if (mc.isMouseWithin(Left))
   cameraX+= scrollSpd;
   if (mc.isMouseWithin(Right))
   cameraX -= scrollSpd;
   if (mc.isMouseWithin(Top))
   cameraY+= scrollSpd;
   if (mc.isMouseWithin(Bottom))
   cameraY -= scrollSpd;
   Thread.Sleep(10);
   ScrollMap();
   }

   private void DrawCameraCoords()
   {
   spriteBatch.DrawString(simple, "X: "+cameraX, new Vector2(5,5),Color.DarkCyan);
   spriteBatch.DrawString(simple, "Y: " + cameraY, new Vector2(5, 20), Color.DarkCyan);
   }

   }
}

2. Finding the tile coordinates.
I was thinking of a way to find the tile's array counterpart with the mouse's and camera's positions. I couldn't figure it out. Maybe you have an idea for this? Undecided

3.How do I make the scrolling rectangles work when the game is fullscreen? Undecided

Thanks in advance!