Weird overheating issue - runs hotter when heater is on

I built a stroker motor last summer and my '90 XJ has been running flawlessly. I have a 3 core radiator, high-flow thermostat (with a bleeder hole drilled), high flow water pump, and hood vents, and it has always run at a constant 195 on both temp gauges until recently. I also switched the electric fan to a toggle switch, but I never have to use it unless it's a hot day and I'm running the AC.

Recently, it has started to run hotter - about 210. I checked my coolant level and it's good. I did notice that if I turn the heater control to heat, the temp will RISE about 10 more degrees - which seems REALLY weird to me. Temp will also rise a little more under load.

I can understand a bad fan clutch, head gasket, etc., but I just can't think of any reason why it would run hotter when I put it on heat. Any ideas? Thanks!

Is your XJ the one that has the X valve that controls the heater?

Right now I would think your thermostat isnt operating properly. I think (at least on the 96 and newer) when you turn on the heater, it opens up the thermostat. Maybe when you turn on the heater something shorts and it actually is keeping the thermostat closed or not opening it all the way when it needs to be fully open.