http://www.airsoftcanada.com/showthread.php?t=94413
All your answers are there, no really, read through the entire load of pages, yeah you could be lazy and ask someone to do it, but its all there in one of those pages. That thread is what I consider to be one of the most educative threads you can read even before firing off a single round on a TM HI CAPA.
Stock TM guns are VERY VERY efficient, HOWEVER, every modification or upgrade has its own ups if you're careful and downs if you're lazy.
Now to be helpful even more: it might be your valve knocker, but you'll have to read the thread to get full diagnosis.