'91 3.0L oil pressure drops out

I have a the 3.0L in my truck, it was rebuilt by Toyota 8K miles ago under the head gasket recall. When I start it up after it sits all night and gets cold, it has plenty of good oil pressure on the gauge, after it heats up a bit the oil pressure drops almost completely out, on the gauge at least.

It doesn't knock or tick at all and I can see plenty of oil moving around in the head when I take the oil cap off, when I rev it oil will fly right out of the cap hole with the cap off. This is when it's hot and the gauge shows almost no pressure.

Is this most likely the sender, or could my oil pump be shot? I don't think an oil pump would just cut out like that, if it was bad it would just be bad all the time right? Would Toyota have used the same oil pump on the rebuild that came off the old engine? It was rebuilt by them at 188K and now has 196K.

I have heard that the pressure senders do go bad on these things, and if they used the old one when they did the rebuild it's not hard to believe it's bad. I just hope it's not the oil pump.

Any suggestions would be great.

Thanks.

Once it's warm does it not register at all? (ie, no needle movement)

It will still register when warm, but barely. At warm idle it is completely dropped out on the gauge, when I start to go the needle will move up to the Low line on the gauge, but this is the Low level of the normal range on the gauge.

This happens after only a couple of minutes of driving, so the oil isn't super hot to the point where it would lose that much viscosity.

I can see plenty of oil moving around in the head when I take the oil cap off, if I rev it high enough it will shoot out of the cap hole, so there must be decent pressure.

It was rebuilt 8K ago, so I have to assume everything was cleaned, like the bypass on the oil pump, but they probably used the same old pump that was on it.

Do the senders go bad at all, and would heat make them crap out, or possibly once the oil gets a little thinner the sender cuts out? The needle on the gauge is in a perfect spot when it's starts up cold and is at the high idle, it's slightly right of the middle of the gauge towards the High side.

Thanks,

Mike