The Nissan Path

I've owned a 1993 SE for a little over a year now and have been experiencing heavy jumping/hesitation problems about 20 minutes after start-up. Helps when i turn heat on full blast. Coolant levels stay the same and i can take the radiator cap off with no problems after a 20 mile trip (no-leaks), new cat and exhaust last week to no avail. Temperature gauge jumps frequently but never passes half way point. Car becomes inoperable about 45 minutes after start-up with an absolute loss of power (car runs perfect until reaching normal operating temp). Seems to hesitate the worst between 2000-2700 rpm. Please help...I'm very mechanically inclined but i just cant put my finger on it. It seems to have gotten worse since the summer started (only happened once in a blue moon during the winter)

Common kind of problem in Pathfinders, but many things it could be in emissions, sensors, etc.
If you suspect a heating issue and not sure , if you have a infrared heat gun I would warm it up to operating temps and shoot it with the temp gun to get an idea.
Did you check the codes to see if it is giving a code ? I know if you do not have a good service manual and lots of patience, you can spend a lot of time time chasing your tail on them !
Process of elimination parts buying can get pricey !
The temp gauges or not so dependable in these like the gas gauge is not, so I would start by knowing if it is heat related and go from there.

I'm just thinking off the top of my head (and it's early so I may still be foggy) but if the temp gauge is jumpy and the truck runs well until about 20 minutes into a trip with hesitation problems I would have three items to take a look at.

1) water pump bearing (binding under higher heat conditions)
2) thermostat could be binding ( rusty bits floating around in the coolant)
3) blockage in the lines because of rusty / crusty bits floating around..

1 and 2 are a pain to fix but two flushes is a less expensive way of finding out whether there is anything physically wrong with the pump and thermostat. If none of the above fix the problem then I would be looking at that thermo-switch thing over the right bank valve cover....