I've posted about my 1990 3.1 before, and so most of you know what it's doing. for those of you who don't, here's what it does: it'll start and idle, and then when I give it fuel, it dies. there was a time once when I could rev it to 3000 rpms before it died, but mainly it'll only idle. Here are the things I've replaced:
-MAP sensor(engine code said to)
-MAF sensor(engine code)
-IAC unit(thought it might be that)
-fuel filter
-checked fuel pressure(OK according to Haynes manual)
-put in new coil packs+ignition module(a freind recommended it)
-it also has new plugs and wires

So, I'm wondering, could it be the ECU causing the problem?

If the oem service number on your ECM is 1227727, then there's a good chance it's your issue. This service number is a high failure part, and will cause random dying/no start/no power issues.