I have a shower/tub with a Symmons Temptrol valve with integrated diverter (s262body)

If I turn it all the way to hot, it seems to be fine for the first 20-30 seconds as it comes up to temp, but then quickly drops off back to just lukewarm. It then fluctuates temp, but never really gets up to hot again.

Other showers and sinks after this one in series from the water heater perform normally.

If I remove the cartrage and just turn on the hot water, the flow from the pipe seems fine (hits far wall of tub). When I put the whole valve back together, turn it as hot as it will go, and then turn off the cold water, i just get a low and variable flow of hot water out of the tub faucet.

It is not the hot water limiter, as that screw is flush on the inside. I have replaced the cartrage (Symmons TA-10), hot and cold seats (Symmons TA-4), and the applicable washers from the washer repair kit (Symmons TA-9) all to no effect.

Any thoughts before I rip this thing out?

Thanks for taking a look :)

Edit: Response from Symmons below (15 mins reply time!)

The only part of the Temptrol valve responsible for and in control of pressure-balancing is the TA-10 spindle. No other component of the valve controls pressure-balancing.

Nothing in the valve knows what temperature is, it only balances the water pressures being fed to it. As you replaced the spindle, you have effectively ruled out our system causing this behavior.

If you are getting full 360 degree handle rotation plus about an additional quarter turn, you are allowing the maximum amount of water pressure feeding the hot inlet of the valve.

Lavatory faucets will always get hotter/colder than a shower valve. Shower valves are always tempered mix between the two pressures feeding the valve.

I can ship you a new TA-10, but I cannot guarantee that this will change anything as you have already replaced it. There may be an issue between the hot water source and that specific valve. The other fixtures in the home do not completely rule out an issue between the hot water source and that specific valve.