Chevrolet Repair: engine hard start,loping,hesitating,dying, vacuum leak, chevy 1500

Question
I own a 88 Chevy 1500 2WD 350. My engine first started dying when I put it in reverse. After a week or so it was somewhat of a hard start(took about five second to start,not norm. for this truck.)Replaced fuel filter. Check engine light had come on which the code read oxygen sensor and Idle air control.Replaced both of those and did a tune up. Wires,plugs, Distributor cap,rotor. Lights still came after a week, ran a little better for that time. then started to act up again. worse this time and especially when A/C was turned on. I have taken it to a mechanic to run a diagnostic. The only thing after three hours he could tell me is that someone adjusted the throttle screw and the timing was a bit advanced. He readjusted both. Still runs like crap. Parts I have replaced since. Computer,EGR,throttle sensor,pcv valve. throttle body gasket.A code did appear one time and nobody can tell me what the part is. It read opti-spark EST. Since replacing the computer no check engine light for three days now about 150 miles worth of driving. Not sure what to do know can you please help? Thanks! James

Answer
Sounds like you have vacuum leak of some sort.  If the O2 sensors and idle are off, you could have a vacuum leak causing the sensors to be out of values.  Also if you had an opti spark code, that would mean that the coil, or distributor module is bad.  Try replacing the module, and coil first.
Jimmy