Three things.
1. Don't reinstall minecraft, force update (unless a HUGE mod you have gotten and replaced a lot of files as well as minecraft.jar)
2. Get rid of the texture pack and try again.
3. See if its your internet.
I got the same sort of problem, I could get on but not hold a connection.
If your using the magic launcher try launch minecraft normally, if you cannot start minecraft normally due to java being a bitch,
Copy the minecraft.exe to your desktop, make a TXT file then copy this in:
echo off
cls
echo /
echo /
echo Starting minecraft..
echo /
echo /
title MC Console
java -Xms600m -Xmx600m -jar minecraft.exe
echo.
pause
Then save and close the window, and rename it to Minecraft.bat
Then start the Minecraft.bat and try join, if it doesn't work for you or you are using a mac and java is messing up, your screwed as I don't know how to use a mac and never will use it.