The car has been running fine for about two months since engine was transplanted to another STD chassis. The car has had 4-2-1 header, oe cat, oe type pipe and muffler. Other than this, the car is stock.

Initially the engine has hesitated on braking as I pulled into the gas station. After refill, it started fine and I have arrived to my place of work. Funny thing is that fuel gauge needle did not come show full indication even though it was filled. As I was making right turn into parking lot, the engine hesitated. In the lot, I made left turn and it didn't hesitated.

Drove home (28miles) and it run fine whole time. On the way back to work, the engine started to act up on highway, going straight. It started as if the engine is misfiring then slowly lost power then it died. Water temp gauge was normal. Quickly I turn on the ignition and the engine started fine. Drove for a mile or two with hesitation and eventual stop. It took several stop and restart to get to work. Check engine light came on during this trip. It was for O2 sensor.

Replaced sensor with new but the engine operation is still the same.
Replaced ECU but still the same engine operation.
Replaced distributor but no effect.
Replaced exhaust header, 4-1,test pipe (installed as temporary), new muffler, and still the same.
Checked valve timing, timing belt and it is good.
Checked fuel pressure and it is about 40 psi with regulator disconnected.
Checked plugs and they are firing on the test bench.
I was watching dash-pot and tandem valve diaphragm operation while engine is running and they were moving. I really don't know the movement I was watching was correct move or not,,,,,.
IACV was checked and cleaned when engine was transplanted, about 1000 miles ago.

I have not checked cylinder compression at this time, but it is hard to think it will go bad this quick. The engine has new head gasket and new rings with honed barrels.

I need to get this car going again since I am stuck here in my office. Can some one give an idea to fix this?
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Checked IACV and it is clean still. The coil has 11.6 ohms of continuity with no fault to ground.
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Checked TPS and it reads 0.678v, closed, to 4.241v, open. Should it be 0.5v to 4.5v exact as I read in other thread?
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It seems like the cause was due to intermittent fuel pump operation because it run good after changing the pump. Main relay was looked at before pump replacement. Solder joints looked good but re-soldered anyways. Test drove for couple of miles and it run fine.
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Drove the car for 1/2 hour. No issues with engine operation. Running with test pipe is something I've done for the first time. It stinks bad. Really bad. The pipe will be replaced with cat as soon as current fuel in tank is used since I mixed AVgas with auto gas.