need more info.what boost at 1500 to 2000 if any?now and before the tune.What timing now and before?it dont take much change if the working compression was borderline before then something is different at that rpm than before such as more timing, fuel delivery,octane,engine temp,boost,engine load etc.air intake charge is slower and hotter at lower cruise rpm.have u done all the little things such as using a colder plug with the top electrode barely reaching over the center electrode and the top electrode being rounded off around the corners, as wide a gap as possible with a good ignition system valve coating etc.What is the dish piston how far in the hole what shape what cc head ?whats gasket thickness?bore stroke and rod length?and what are the cams intake and exhaust valve opening and closing timing.with all that going on i would get transmission problem corrected first because it could be causing to much or not enough load on engine causing timing,fuel delivery or boost to not be in sinc.just my .02 cents come on guys lets make this a learning thread lord knows I need to learn some stuff.
Clyde