Hello: Here are a few random thoughts. Are you getting consistent temperatures from other fixtures? If so, it’s a shower valve problem. Is the shower valve pressure and/or temperature balanced? This could be playing into the trouble. Does it have an adjustable internal stop to prevent scalding by limiting temperature? This could be re-set. Is there any chance some debris from plumbing in the new heater made it’s way into the shower valve? This could restrict movement of the pressure balancing device and cause trouble.
I think I’d disassemble the valve, clean it out, flush the lines with the shower guts out (messy! :P) and see if it helps.