I had never heard that issue about the temp probe, but if it's true it's fascinating. I use the same probe, and I also have weird temperature issues in mashing.
(Also, I use it while boiling, and one thing that happens is that it will routinely show the wort at over 220 degrees F before any actual boiling happens).
J-Pzzel, I have the same mash setup as you (10-gal cooler w/ mesh tube). Pre-warming the tun is a huge help in limiting your initial heat loss. (Just put a couple gallons of pretty hot tap water, even, and put the lid on, then slosh it around a little and let it sit for 5-10 mins). When I did this I found my heat loss at dough-in was exactly what the recipe predicted (about 7 degrees), rather than losing much more to the walls of the tun.
To add a question to this: Can I just use a straightforward formula/ratio to when calculating how much water and at what heat to add, when aiming for a particular temperature?
For example, using numbers from yesterday's brew: I had about 11 quarts of 122 F mash in the tun, and I added back about 4 quarts of boiling (212F) decoction. I was aiming for 152.
According to the weighted average, this would raise the average temp of the mash to 146 (still too low), and in practice it went only to 140. So really, I should have added at least 5.5 quarts of boiling mash, and to be safe maybe 6 quarts.