I had this exact same problem , and recently too. It turned out my oil pump took a sh*t and left me with some locked up lifters and a messed up push rod. It would be around 38 psi cold then to around 5-15 warm. Finally i could hear the lifters clicking really loud and i paid the price for delaying.
I thought my gauge was bad (i have an external sun pro gauge), but it seemed to work ok when cold so there wasnt any air in it.
89 GT Hatch: Custom M90 Supercharged w/ Ported Trickflow Trackheat Heads, Stage 1 cam, Streetheat lower / Sheet metal upper, Tweecer RT & PLX Wideband, 75mm TB, 1.6 Roller rockers, 3.73s, MSD 6a, Canton 7qt Pan+ Windage tray, 36LB Injectors, 200 amp Alternator, 75mm Pro-Mas MAF, 190lph fuel pump,Spec Stage 1 Clutch,SouthSide LCA's, Pro 5.0 Shifter, Steeda CC Plates / Springs, FMS Aluminum Driveshaft, New T5 w/ 2.95 first gear, Underdrive pullies, Zirgo 3300Cfm Elec. Fan, AFCO alum. radiator