I'm curious to see if I'm right. You said you replaced all the basic stuff already. So my guess is still check the compression next. A tester is realy cheap at Autozone and only takes 5 min. I purchased an ls with 160,000 miles. Shortly after startig to rag on the car, it started cutting out sometimes under excelleration. The rest of the time it seamed to run fine. So I thought it was fuel or ingintion cutting out. Shortly after swaping a b20 into the 90 integra I installed the ls into my 90 hatch. The thing turned over great but wouldn't start. After three days of bs, checking wiring, relays, fuel, ignition.... I finaly did a commpresison check. 70 in each cylinder. Paid $160 for Nippon p30 pistons and rings ( raised my commpression for a little bump in power. You can just get the rings and skip having them pressed ), $50 to have them pressed onto my ls rods, $60 for rod bolts, $30 for Autozone cylinder honing tool. I already had a new head gasket laying around. found it easiest to just pull the head and oil pan with the block still bolted in. Honed the block right there in the car. Been running like champ ever since. If you know what your doing it won't take more than two hours to pull it apart and can be asembled the same day you get your rods pressed, or in your case the whole thing should be a one night project. Hopefully it is something simpler but for me it wasn't and I wasted more time looking for the problem then I did fixing it. It could be something like vacum hoses mixed up or crapy addjustable blox fpr that's sticking ...? Sometimes mixed vaccum hoses will make the car have a lobe at idle. Good luck! This is one of my first posts so let me know if I was any help.