GE Profile oven strange temp problem

I have a GE Profile oven jbp90gw1bb. Oven will get up to temp and maintain temp just fine. If I reduce heat (or turn off and then back on before it’s cooled completely) the oven will not maintain temp, even if I switch to broil or convection, which use different elements. It falls down to 170 (the minimum settable temp) and stays there, so it appears it’s still capable of heating, it just wants to heat to 170! I HAVE replaced oven temperature sensor probe thing, no luck. Control board for this oven is only about $25. That’s my next thing to replace. Any ideas? Thanks.