mash temp

I use an igloo cooler that is pre-heated with water for about 20 minutes and then slowly pump water in around 170 while i dough in and stir. What I have found is my mounted temp probe reads around target mash temp but when I vorlauf the last twenty minutes of mash the temp goes up on avg 6...

I use a RIMS set-up for mashing / all gain recipes, and I recently tried (failed) to heat up the sparge water using the same system ... which is where my question comes in. But first, a couple details. My system for mashing is a large insulated cooler with a spigot, mesh filter (false bottom)...