Vehicle: 98 4Runner 4x4 3.4L 170,000mi

The Story: I recently purchased an '81 pickup that has been stealing the show (both maintenance and driving time) from my old new 4Runner. So, I have been driving the 4Runner rarely with routine 2-3 week periods between drives. Recently, the engine started stumbling under moderate RPMs (2,800 and up). Prior to this the engine would stall after driving for a while and coming to a stop. This caused the check engine light to come on (P0125 - insufficient coolant temp code) which fixed the stall issue but not the stumbling. I first noticed the stumble while driving but it also does it while stopped and in neutral. I think it is fuel related, but I also don't want to throw a bunch of money at random parts with no fix. In my attempt to diagnosis the issue, I have seafoamed, replaced spark plugs, fuel filter, and upstream O2 sensor. The engine runs fine at low RPMs and gas mileage seems normal.

Could the coolant temperature sensor cause this? Has anyone replaced one of these? I have read that a fix for the P0125 code is to replace the upstream O2 sensor which has seemed to fix at least the stall issue. Would a bad fuel pump or injector cause this? If it is an injector, how can I diagnosis the faulty unit?

For those that have read, here is a pic of my mistress:

 

It's probably a sensor but I would start with fuel starvation. Weak pump or fuel filter clogged. In cases like this it might be worth a diagnostic check at the dealer. Most auto part stores will pull codes for free which it sounds like you have done already.