I am having trouble with my 85 535 stalling hard, backfiring, and jerking the car (like someone grabbing you by the shoulders and shaking forward and backward). It happens at about 1500 RPM in 1st and 2nd (manual trans) and at about 2200 RPM in 3rd. If I push hard through the rough spot, the car will accelorate and have full power, and it will also idle and run at low RPM w/out stalling. I have changed fuel filter, MAF, Throttle switch, O2, cap/rotor, wires, plugs and a partridge and a pair tree. The only thing I haven't changed is the Fuel Pressure regulator. There seems to be strong vacuum, with a noticable decrease in idle when I pull the dipstick. There are numerous exhaust leaks down under, and there are noticable clouds of smoke coming up from the rear of the engine when I run Sea Foam into a vacuum line. (expect smoke with Sea Foam, and it is supposed to clean out carbon deposits). The car is ringing in at 290,000 miles, and I haven't done anything to the intake or valve adjustments since 162 K where I bought the car. Ideas? This has been a great running car, but this one really has me stumped.