First, make sure your not using a heavy weight oil.
Second, throw a guage in, i took the guage sender out and put a mechanical guage in, Theres really nowhere else to put a guage in. Test the pressures at cold and temp, these specifications are in the toyota manual, and possibly the gregories aswell. (i will look these up for you if you dont have either)
I have heard alot of people having problems with the guage being faulty. So this may be your problem. Toyota want i think $60 for the sender and about the same for the guage, so alot of people just end up fiting a good quality aftermarket guage. The EDIC relies on the oil pressure switch rather than the guage so you can safely remove the guage sender without worring about your engine cutting out.
If this doesnt bring light onto your situation, drop the oil pressure relief valve, near the front bottom drivers side of the block, big plug, about 21/24mm? Described in both the gregories and toyota manuals.
There is a plunger and spring behind the plug, to get the plunger out, pull the plug and spring out, then turn motor over, not for long though as oil is gonna poump out aswell, the plunger will come out and fall into the dish you have under it, it may bounce off your spring and end up somewhere else so have a look around
If you have a dowl about 12mm you can shave the end of the dowl down to about 10mm and then it will slide in the back of the plunger and that will drag it out. Turning the motor over is the easy/messy way tho.
Check for wear on the plunger, you can get cheap replacement springs and plungers from aftermarket sources, or you can get them for less than $20 at toyota.
If there is alot of wear on the plunger, it has probably worn a step inside the bore. Get a timber dowl about the same size as the plunger, (12mm?)cut a slit in the end, wrap some wet n dry around it (stick it in the split then go round 3 times or so, to make up to the right diametre), then spin it around inside the bore, your trying to remove the step only, you dont want to increase the size of the bore too much as this could cause more chance of the plunger sticking.
Alot of people have success by just replacing the plunger, you could try that before linishing the bore.
Normally if your getting low pressure the spring in this relief valve has compressed, but if your getting high pressure, its more than likely the plunger sticking. (if youve ruled out other possibilities)
The high pressure shouldnt cause a problem in your engine, if it gets too high the oil filter will let go which is incredibly messy and inconvenient, but it really shouldnt do any damage to the engine since the EDIC will detect the low oil pressure and cut the engine within 5-10 seconds. The high pressure will normally push the rubber seal of the oil filter out, or the seam of the oil filter will split.
Valvaline had a writeup about this exact problem, oil pressure releief valve sticking, in the IAME magazine my old man gets. I thaught it was valvaline covering thier asses for thier crappy filters at first, but after investingating the problems with the 2H, i can only imagine how many problems they have in lesser quality engines.