Partial Mash Temp Issues

I've been doing partial mashes for about a half dozen batches now. I use a 2 gal cooler with a grain bag. I've been getting some very odd temperature behavior in the mash tun recently. (I use two floating thermometers, both have been calibrated at 32F and 212F. I also preheat the tun with ~1qt of boiling water.)

Here are three recent cases:

1. 3.5lb grain, 4.5qt of 166F water = 153F mash (perfect!)

2. 3.5lb, 4.5qt of 163F water = 142F mash (expected 150F, WTF?), added 0.5 qt boiling water = 149F mash (good!)

3. 3.5lb, 3.2qt of 146F water (protein rest) = 120F (should have been 131F), added boiling water = 129F (OK)
Added 1.5qt boiling (sacc. rest) = 142F (expected 154F), added small amounts of boiling water a couple of times, only got to 149F.

I want better control than this. Anybody have a PM/cooler process that's really accurate on temps? Maybe put water in first, get temps right, then add grain?

(By the way, case 1 above was Yooper's Dead Guy Clone. We cracked open the first ones last night...easily the best beer I've ever made. Well done, Yooper!)

Exactly, use your strike water to preheat the tun. Add grain after temp has stabilized.

Also, don't ignore the temp of the dry grain.

I have been doing partial mashes using a Presto Kitchen Kettle. Basically a Fry Daddy with an adjustable thermostat. Works GREAT! I can mash up to 3 pounds. Just have to stir consantly until next rest temp. is reached.

I also do PM brews in a 2 gallon. I don't use a grain bag though. It has taken me about 6 batches now to figure out the temps in it, but I pretty much have it dialed it. The two things that I do that really make the difference is that I preheat the tun with hot tap water. I fill it all the way to the top. I let this sit while I am waiting for the mash water to heat. The other thing that I do is that I put a towel on top of the lid. This made the biggest difference as a lot of heat is lost through the top of those things.

Like others have said make sure you take your grain temp into account. I use the spargepal app for the iPhone to get my "target" temp and then I add 4-5 degrees to that number. You can also use the formula in How to Brew, I get the same results. This gets me within 1 degree of my target temps.

Hope this helps.

I'll second what others have said: Pre-heat your mash tun with your strike water, adjust the strike water temperature for the temperature of the grain, and stir the mash well, giving it a couple of minutes to reach a stable temperature. I use Beersmith to calculate my temps, and usually end up within a degree or so of my target. Having some ice and a cup or two of boiling water nearby is good, just in case you need to do some adjusting.