hard to start

i have a 89 525 that is hard to start.if i just try starting it i starts then dies starts then dies. if i hold the gas down then start it, it starts really rough, but it starts. then once im driving it, it will somtimes want to die, then it backfires, then its fine for a while. ive replaced the plugs and fuel filter. this helped a little. but it still somtimes has thi ssame problem. this is driving me crazy. any help would be great.