'86 NA, small streetport S5 drivetrain swap (details on a thread on my page). Car has been bulletproof on this setup for 3 years. Spun a bearing, hit a quick rebuild, new hard seals and rotating assembly. Hosuings/Irons specced out well and survived. Motor's in the car and ran solid for about a day, till I decided to take it on a longer break in drive. About an hour in, car bogs down, smokes, and drops to one rotor. pull plugs fearing the worst and crank for a poor man's compression test. Sounds even. Rear rotor plugs are wet, so I clean and reinstall and was fine for another two hours or so. Hot starts just fine multiple times, seems like normal. After time passes though, does it again, this time will only start on the front rotor no matter what I try, plugs are wet on the rear and even idling on one rotor it's shooting fireballs and unburnt fuel out of the rear rotor tailpipe (RB true dual exhaust). Fast forward a couple days to today. Disassemble both fuel rails, test all injectors, test primary signal with a noid light, and it all checks out. Clean plugs, restart the car, and runs perfect again. Let it idle for an hour ish, trying to make sure it wouldn't die on me again, no problem. Go for a test drive and it dies again in 30 mins and won't restart. I'm truly dumbfounded. Any ideas welcome.
Things tested:
-Spark from all 4 plugs (when not obviously fouled)
-Coils
-Primary injectors when CAS spun
- Secondary injectors individually (can't figure out a way to test harness itself, think maybe the issue could be a short there?)
-Compression (again no exact numbers, as I don't have a RCT around, but is even by sound and by piston slowmo method.)

 

sounds like an injector is sticking after the engine warms up. might be time to swap them or send them out for a good cleaning.