Do you have the risers with screw in fittings? if so make sure the fitting is bottomed out inside the pipe, I have seen it before and not just with stainless. None of tem make their own fittings and the pipe threads seem to be very inconsistent. if it is bottoming out just cut about 3/8" off the end of the fitting and reinstall it.

Hillbilly thanks...one more thing to check on Sunday.

I hope it's just the thermostat but better to be sure.