I can't figure this out

[sweet88coupe]

just me,

try the MAP sensor, my 88 had somewhat the same problem and the vacuum line was off the MAP sensor, but you said you checked all the lines, replace the MAP sensor for kicks!!! I think that will cure your problem.

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'88 coupe
351W (.060" over)
trickflow "R" series heads, 1.6 ratio crane gold series rockers.
Miloden 7qt oilpan
trickflow street series intake, with 70mm throttle body, and 77 Pro-M mass air meter.