From what I hear my problem is a common-ish hardware defect: the volume keys stop working. Not only that, but they stop working by constantly pressing down. This causes a number of homebrews to crash, for example the FreeCheat cheat device. This is because there are functions (probably originating from SCEM cheatmaster) that cause the thing to wait endlessly while buttons are pressed, that have VOLUP and VOLDOWN in the key mask.
To fix, decrypt the prx in question and look for:
E8 FF BD 27 F0
E8 FF BD 27 C0
This was found by compiling scepxmb, editing the key mask in src, recompiling, and comparing the differences with vbindiff. These particular values probably only work on things based off of scem code.