Engine Problem

1988 S10 Blazer 4.3 Ltr V6 -- When started, the engine runs rough and stalls. Will do this several times before it starts and runs smoothly. Acts as though it is choking out.
Gas mileage has dropped from 20 mpg to 16 mpg. Also, after running under load, the engine will restart fine but will stop when put it gear if restarted sooner than about 30 minutes after it was stopped. If restarted sooner than about 30 minutes after having been turned off, it will repeatedly stall out when put in gear for about 5 minutes, then when it does start running in gear, it might stall out again during the next 5 minutes or so. Acts as though engine is vapor locked. After that, the engine runs fine again. Cannot figure out the problem. Have replaced the spark plugs, plug wires, fuel filter, pcv valve, EGR valve.