Mash temp spikes - can't control

I've gotten a new brew-pot, a Blichmann 10 gallon with false bottom and built-in thermometer, and I'm struggling with mash temperatures. When I try to raise the temperature from, say, 122 to 130 the thermometer (even though fully immersed in the grist) will stay at around 122 as I stir, and then suddenly spike up to 140! I think the false bottom is part of the problem, but if I take it out I have to pour the whole mash out into another pot, replace the false bottom, and then sparge, which seems a hassle. I've tried draining off a gallon as it heats and pouring it back in, but this doesn't seem to help.

Ideas?

Thanks!

 

Those types of thermometers sometimes stick - you need to tap them periodically to "unstick" them.