Hi guys, I need your help, so every once opinion is appreciated, and thanks ahead! I have 1987 GMC G20 series 2500 Vandura, with 305/5.0L TBI and 400HD THMatic transmission, full size van. The truck starts just fine, but idles at somewhat higher RPM range as it should (even warmed up), around 1000. It would idle steady but then all sudden as if someone is pulling a life plug on it, or should I say turning of its ON switch, it begins to shutter, shakes, puffs, basically all sudden begins to struggle to stay up, and eventually usually would choke on itself and die, with check engine light on. Code 33, High MAP voltage, low vacuum. It will start right away again w/slight backfire, since It was just chocked on gas, idle another half min and die again, or sometimes what it does is after all that struggle that usually lasts about 30sec to a min, it comes alive j/as fast as it begins acting up and then would idle again w/slightly high RPM but steady for another min or so, then does that struggle thing all over again and usually stalls eventually with same Code 33. The truck is in our fam for over 18yrs. When it runs it runs gorgeous. We have rebuilt its engine about 3yrs ago, so it only have 6k on it, with RV cam, rebuild heads, pistons, rings, bearings and all that…! Its TBI was also rebuild, along w/its manifold which was steamed and blown off from inside out. About a year ago we had similar problem w/it, and at that time after testing various systems I found the problem which was MAP sensor acting up. Right now as I type to you, the engine already have been given new MAP sensor, set of spark plug wires, spark plugs, distributor cap and rotor, ignition module. I have replaced all these in process of searching for a problem, as I have life time replacement on those parts anyways. I have also checked its compression to be sure that its mechanically sound, and it pumped out on average 5% below stock # which I believe is awesome, all cylinders pushing 168-175 on me. The reference Voltage of 5Volts is also there at all the sensors, including MAP. Engine pumping out 20mEr of Steady Vacuum on idle. Distributor is not loose, so I have not yet tried to re-adjust timing since it was all set up nicely last time before the problem occurred. Guys, im a little stumbled on what should I do or look at next, so please, any of ur advise is highly appreciated. I can tell u that I have not entirely checked everything, I think I left out IAC, and TPS which I will look into shortly, however if u have any suggestions to what you think I should do next, please let me know. Thanks!

Sorry for bugging you guys w/problem in regards of the Truck, i know its Bonnie forum, but ppl at GM truck forums over at other websites are kinda dead, no help there, and i know that u here specially some of you are always very helpful. Thks ahead.