You might have a bad thermostatic valve. Time for a cartridge replacement, I think.

thank you- is that specific to the bathroom? would that be why it doesn't happen in other bathrooms ?
When you say thermostatic valve forgive my ignorance but that is specific to the location and not the whole house system? If so sounds right!

Have you had work done in the house recently? It founds to me like you are getting a cross connection or flow. Turn off all the cold supply valves and do flow/temp tests on the hot. I have seen cross flow in washing machine valve set-ups at times. I have also seen the situation you describe where the H/C supply lines were crossed on a new shower. These can be a pain in the neck to find. Get a method and follow it so you know what is happening at each faucet. It could very well be the shower valve as previously noted.

We have not had work done recently.... well about 6 or 8 months ago new water heater.... this is definitely beyond our ability. We do have a good plumber but I wanted to get an idea of what it might be, if it was something that was just simple enough like flip a switch. Well I can dream on I supposed. Thank you for your help. Will post back and let you know what it is.

Yes, I'm talking about the on/off valve in that shower. (The handle operates the valve.) That valve may be thermostatic or pressure-balancing, which means there are parts that can be replaced.