My '87 T-type is overboosting to 24-26 PSI (with both different 17lb. street chips that I've tried). When I put in the Stock chip- it overboosts to about 20 psi. If I plumb the wastegate directly to the compressor, the Boost control is fine- I can adjust it all the way down to 10 psi. I've already replaced the Boost line solenoid and boost control hose asby.
There are no vacum leaks- all hoses are tight and the car pulls 16" vacum at idle.
Could this be due to a bad MAP sensor (does the MAP tell the ECM how much boost the car is making, thus causing the ECM to pulse the boost line solenoid?)
The MAP is only for the dash guage and is not connected to the ECM in any way. On digital dash cars, it is replaced with a switch for the boost 'lights' instead.
You may have a hose assembly with the incorrect orfice size. This was a problem with a batch of these parts. See http://www.li.net/~rsalerno/tta_wastegate_y.htm for an illustration.