I am having some of the same problems. I replaced the vac lines and vac mod on mine about 1 1/2 months ago. I purchased my replacement modulator from Advance Auto and, it was smaller with a red stripe (they said "THIS WAS THE ONE"). Not too sure about the diferent color stripes but, it has something to do with pump pressure and shift firmness. First test drive and it refused to shift until HIGH RPMS. Called Advance-- guy said I needed to adjust (loosen) screw in vac mod. Seemed to improve matters but, still not right. Proceeded to adjust via the vac regulator at IP. I have decent vaccum but, did not get shift vs. RPM corrected. SO, I went back to adjusting at the modulator, per a thread here on SS. When I adjusted the vac mod to get shift timing / rpms right, the trans started slipping. NOT GOOD! Out of curiousity, I went to O'Riley's. They had, in stock, the small vac mod just like Advance AND, a larger vac mod (white stripe) just like Unc. Sam installed. Bought the larger unit and installed with out adjusting anything. Test drive results= shifts like it should. However, I'm afraid that I may have damaged my tranny by trying to adjust and use the vac mod from Advance. MY ADVICE-- MAKE SURE YOU'VE GOT VACCUM AND GOOD VAC LINES! Then go to O'Riley's and get the big vac mod.
