Well i'm a noob, and i dont know too much about the central locking system on my car (and sorry, no diagrams), so you prob know this already...
I noticed that a feature of these cars is if you leave the keys in the ignition (all the way off, but still in the lock) and try to use the armrest switch to lock the doors, it locks then immediatley unlocks (in the same way you describe), presumably this is to stop old gradnpa from locking his keys in his cressida , perhaps then the problem could lie with the ignition part somehow?