Background:
My 134a-retrofitted A/C has always had the habit of cutting out on really hot days, though it works great when the temps are around 80 degrees or below. Always thought it might be some kind of temp/pressure-related issue. Recently, it's stopped working altogether.

With the help of a friend (who's a helluva lot better at reading elec diagrams than I am), we've established the following:

-System pressure is a little over 100 psi static on the high side @ 70-80 degrees ambient temp.
-Fuse #9 (hot at all times) leading to compressor control relay tests "good".
-Compressor control relay itself and all associated fuses tested "good".
-Fan clutch is "good" and aux fan low & high both work when they're s'posed to. Engine temp never gets higher than 12-noon.
-When I jumper the black/gray wire coming off the pressure switch (the one which leads to the compressor) to a 12-volt lead, the compressor springs to life, engages the aux fan, and provides 45 degree air at the vents.
-If I do this long enough, the aux fan kicks into high speed, as I gather it should. I feel like I've tested all the "right" things.

Could it be anything other than the pressure switch on the evap/drier? Do I have to evacuate the system to test this thing?

Any ideas would be appreciated,

Tim "sweaty bits" Goodell
'91 535iM