You know, you could be right. Now that I think of it, I did hit the dyno again to fine tune some trans issues I was having between track outings (shift points and TC not unlocking when letting off the throttle, causing it to act as an engine brake upon deceleration.....very annoying!!!). Perhaps some of my issues between the 1-2 shift were cleared up then? Bottom line though, if the boost isn't stalling, then it's not a belt slip issue.
Mine was an issue however, since I'm running a 2 3/4" blower pulley on my M90. Even with the 8-rib set up, it's holding on for dear life to what little of the belt surface contact it's making. Lol
Your larger M112 probably doesn’t have that problem, since you’re able to run a much larger pulley (which allows you to maximize belt traction) and still move decent volumes of airflow.
Roots blowers will normally spike fairly quickly and may climb 1-2psi durning the last 1,000RPM or so of the tack. That's not a bad thing and is often quite normal..especially if you're running stock heads/cams/etc. It means it still measuring some amount of restriction, that's all.
It's watching them drop off at the last 1,000RPM of the tack that you don't want. That means that you aren't running enough blower and it's unable to keep up with the airflow demands of the engine.