Check the air temperature sensor. I once had a bad connection, and it was kinda similar. It thinks it's cold, so it runs rich & screws with the timing. It never runs goes into closed loop (using the O2 sensors)... just uses MAF readings & stock lookup tables (which aren't that good for a modified car).
When I took a U-shaped wire & jammed it into the plug, it INSTANTLY ran better. I thought the sensor was bad, so I replaced it... but had the same problem. I put some di-electric greese in the connector, and haven't had a problem since.
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