Car turns over but won't start

Hopefully someone can help me out here. I have a 2003 buick regal ls 3.8l motor 204k . Recent work done New plugs and wires new coils, new front and back valve cover gaskets new fuel filter and fuel pump , new MAF sensor, Both 02 sensors, New PCM ( still have old one and it's apparently good because it didn't solve the po340 code problem. I left work one night and the car stalled out and would crank over but not start. Of course the battery died but I have been using my buick lesabre to jump it.  I had it checked out by a mechanic and he checked the following and they were working, plugs got spark, fuel pump was strong, coils fine naturally, battery was good. The car ran great before this happened the gas pedal seemed a bit tighter that afternoon so I immediately thought the fuel filter or pump which he tested 1st and both were fine

The other day I went out to try to start it again and it wouldn't start so I hooked up my code reader and both the p0340 and p0650 codes that were there before don't show up anymore but then again it only scans for about 15 seconds and it's done. I bought the PCM on ebay and it was programmed to my vin#, my key fob worked right from the start and it ran somewhat fine for a month then this happened . Anyone have any suggestions? I keep thinking I should put the old PCM back in any thoughts on that? i'm thinking maybe something is wrong with it.