Ok... reporting back
I followed some good advice, and decided to check what I could before posting again.
The problem: '94 TS 3800 starts, runs for several minutes, then the RPMs start going down and back to the original value. It does this either at idle or while running. Since I bought the car you could feel the car "missed" sometimes at highway speeds (never long enough to notice anything more than a slight pull), and died from time to time when starting uphill. Now -after spending hours updating the interior, replacing the intake gaskets and buying a new battery- the engine not only misses but actually dies
and it's becoming harder and harder to start after it does. Fuel pump sounds like it's whining more than normal, but I'm not sure.
Already done: changed the fuel filter, built an OBD cable, pulled the codes (QDM1 and QDM4 errors), checked G200 and a couple others. The cable shows some weird values sometimes, but it may be due to it being a makeshift version before I build "the real thing". The problem seemed to start when the car warmed to the "closed loop mode", but can't be sure.
My guesses at this point are a bad fuel pump, a bad ignition module, a bad ground I have not checked yet, or something like a small hole somewhere in the fuel pump intake making it lose suction or something weird like that.
My question: is there something obvious I may be missing? Is there any test I could do to verify the ignition module is not working ok?
My resources are limited so I would really try to avoid the "change and then test again" approach. If I were to choose between buying the fuel pump or the ignition module first, which one should it be?
Thanks!