I own a 2-story house with a hot water recirculation line. Two of the faucets consistently run out of hot water. I changed the internals in one of the faucets, to no avail. The faulty faucets will supply hot water at times, and then a flow slightly warmer, then cold, then a little hot again. I tried shutting off the pump for a few days, also to no effect. To replace the pump would be a problem, and I'm not dying to do it considering the remaining 5-6 faucets work fine. Any suggestions?
The fact that you have 5-6 faucets working fine and only two that don't means we can eliminate problems with the water heater or water pump. The problem must be in the valves or faucets themselves. If the valve wasn't fully open, it's possible that when it heats up, it could expand and interrupt the mix of hot and cold water. The same could happen internally for the faucet. Concentrate your trial and error search on you plumbing system, and not so much on water heater or pump itself.