Hello. I have a 2000 GMC Savanah 1500 conversion van with a 5.7 liter Vortec with approximately 80,000 miles. About 3000 miles ago it started dropping A LOT of coolant through the tail pipe. I bought this van used and noticed when i flushed the coolant out that it looked like it had a stop leak added to it as there was a rubbery film all over the inside of the radiator. i had heard that these engines had a bad history of needing the intake manifold gaskets replaced so i replaced the gasket, flushed and put the engine back together including a tune up with new filters, plugs, cap/rotor, wires, fuel filter, pcv valve, etc. i installed ngk iridium plugs to proper gap. the engine ran well but it still was leaking coolant, so i decided to just buy some more radiator stop leak (Barsleak brand) as apparently the intake manifold gasket was not the problem. Adding the stop leak DID CORRECT THE PROBLEM and the vehicle no longer was leaking coolant through the exhaust. I have just given you some history of what has been going on with the vehicle up until this time. We have put about 3000 miles on the van since then and it was running fine (apparently) until last week when it started running VERY rough. i could tell it was misfiring badly. I put my code reader on it and it was throwing two codes. a random misfire (P0300) and an O2 heater failure. My code reader has the ability to watch real time diagnostics. it showed the #4 cylinder misfiring on almost every stroke(it registered well over 10,000), and the number 3 cylinder misfiring intermittently. On this motor the #4 cylinder is on the passenger side second from the front, and the #3 cylinder is on the driver's side 2nd from the front. I figured with 80,000 miles it couldn't hurt to change out the o2 sensors and maybe that would fix the problem. 2 new (before cats) sensors were installed and it did NOT fix the problem, however the o2 code did go away (so far). I searched for vacuum leaks and could find none. I next hooked up a vacuum gauge and the readings were:

at idle vehicle in park:
irregular needle drops from 21 to 19

at idle in gear:
irregular drops from 18 to 16

The vehicle surges and sputters when in gear. you can here an irregular pattern to the misfires. It feels like it wants to stall out but it has not. In gear and under load (normal driving) it has a decrease in power and you can feel the van surging when it misfires.

I next pulled the #4 spark plug wire off the distributor. the engine ran noticeably more smoother but with an audible clicking sound). In gear at idle the vacuum stayed at a pretty steady 21. once i put the wire back on the engine again started misfiring.

I next pulled the front passenger side tire, and pulled the plug. it appeared ok and was not fouled. i checked the compression on the #4 cylinder only (so far) during 4 compression strokes it rose steadily and topped out at 150. i did not see an issue with the compression.

Does anyone have any ideas as to what can be causing this? I dont have the money to get it to a shop, and i dont have the money to start randomly replacing engine components and praying it will fix the problem. i can try doing a leak down test or simply checking compression in other cylinders? This engine has only one coil. can it possibly be the fuel injection? it has no fuel rails, but rather a set of spyder injectors. egr valve?? cam position sensor?? any help would be greatly appreciated??