When using a cooler this worked fine. When using a SS keg as MLT my temps drop, I usually direct fire to raise temps twice during the mash. I use 2 layers of foil bubble wrap and blue sleeping pad to insulate. Temps still drop. Best results when I also use a sleeping bag over keg AND previous mentioned insulation.
When I add the grains the temps appear to stabilize at the desired temp but w/in 15 min they begin their downward path. Add heat, appears stable then they drop again within another 15 or so minutes. After the next direct fire temps finally stabilize. I never preheat anything, I think I need to bring my strike temps way higher than beersmith says to do, let it stabilize,~20-30 minutes then mash in. I believe the volume of steel continures to absorb water heat.
Drinking Brown ale, Belgian Dubbel, APA, Malty IPA
Conditioning Malty IPA
Primary Bock, Oktoberfest
On Deck American Amber, Pale ale, Arrogant Bastard Clone