I am new to this site. I recently bought an 89 mustang gt convertible with roll cage racing seats 5 point harnessess. The enging has been bored out to a 308. It has cold air intake. I recently put in new battery, clutch cable fuel filter and 2nd gen alternator. has alpine head unit with 2 12" subs. 2 amplifiers one for subs and one for speakers on doors. After driving for about half hour the car backfires in 2nd gear causing it to jerk back and forth and stall at times. At stop lights sometimes the car tries to stay at 1000rpms sometimes I have to rev it to keep it from stalling. Anyone know how to cure my problems.