1996 Chevy Lumina not starting

We recently purchased this van with the 3.4 L motor. In the last 2 1/2 weeks, we have been plagued with starting issues. It will crank but not enough to kick over. Mostly this is when the engine is warm. I have replaced the ignition control module (fun job) the fuel pump (when full). It is getting spark and you can hear the fuel pump engage. The fuel filter like wise has been replaced. This prob. has been intermittent, then all of a sudden you are not driving it for 2 days. It will 'clear up' for about a week and it starts again. No codes are showing up on a scan tool. Ready to call the junk yard or a demolitions team.