Last week my car was dyno tuned and constantly hitting 7-8psi. Yesterday I couldn't get it to go past 5psi. Seems like it hits a limit at 4.9psi and just drops hard below 1psi.

I checked the belt for slippage and also checked for vac leaks, but everything seems normal. I'll probably look for a better leak detection method, but besides that, what are the other possible causes for this?

B18C, JRSC with 3.8 nose pulley & LS crank pulley.

 

Sounds like belt slip

 

My belt from alternator to blower is super tight. However, the belt from crank to alternator is not as tight as I ran out of adjustment room, but it is still within normal tolerance I think.

Can a loose crank-alternator belt do that?

 

yes, a loose alternator belt can do that, because the supercharger is driven off of the alternator pulley, just becareful how hard you tighten those bolts, because i already snapped the tensioner bolt on mine, now it needs replacing.