Static

ok so I got brave today (not really since I have two of everything including the stock exhaust) and I welded a muffler I had off a car that was a flowmaster 4 series.... well needless to say I might as well run with no muffler at all cause this thing is beyond all reason of loud... its coming off tomorrow but non the less I wanted to ask why this is happening... when I am in 4th gear around 5-6k RPM the motor starts stalling like its missing on a cylinder (if it had them but that is the best example I can think of) I dont know if this is cause of lack of back pressure or what but its pretty bad and it is way way way to loud... and also when I am just driving in say 4th gear around 45mph and I let off the gas but DO NOT push the clutch in it starts doing that missing thing again as the revs drop.. like there is an engine break...whats up with my car....