Weird overheating issue - runs hotter with heat on

[JeepFan]

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?