Why after installing a NEW hot water pressure geyser do I get good hot water pressure for a day, then nothing. It only returns after I turn off the element and let the geyser cool down a bit? After a while it stops again. The pipes have been cleaned and fittings changed, but still it continues. This is frustrating. The plumbers are also confused about it?

I am guessing you are speaking of a pressure reducing valve for hot water. I have not heard of one for hot water. Perhaps if you put it on the cold inlet at the water main to the house?
If you are speaking of a water heater, can we have more details? Who makes it. What gallon size? Where is it installed? Was the box opened when you bought it?