I don't think the fluid has been changed since the rebuild.

Seams with cat mod I did it still is slerging. I'm notiicing it at 20MPH just shift into od is this supposed. To happen.

Things to check for surging.

O2 sensor.
Clean the MAF
Check the EGR, make sure the pintal isn't stuck open.
Check your fuel pressure.
Check your spark plugs for proper gap.
Test your coils.
Check for exhaust restrictions.

Good suggestions. I'm on it now I really want this thing in tip top shape for when your tuned pcm arrives.