I've found that raising the temp from ~125 to ~150 to go from protein rest to mash temp is really hard with brew in a bag (stove top mash)...
First, I have to constantly stir with heat on, or pressure builds up under the grains, and it sounds like its going to explode.
Secondly, the temperatures seem to have pretty severe pockets, and I've gotten more than 5 degree differences in readings depending on where I put the thermometer.
I noticed that Papazian recommends adding hot water to raise temp, instead of using heat. Is that what you do? The problem then is that often I'm already at capacity in my kettle for mashing.