10 gal Cooler and heat loss

[iparks81]

heres the scoop
Ive got a Rubbermaid 10 gal Cooler/mash tun. Works Great!
But the last 4 brews have been brutal...Ive never lost any amount of Temp through the 60 min/90min/120min mash times...like maybe 2 Deg on a 120 min mash time.
recently, within the last 4 batches of beer approx a week and a half (Ive been brewing my ass off recently before the summer heat kicks in)
the Cooler has been losing 5-7 degrees within a 1 hour mash...Im having to pull mash out (liquid) and boil to bring the temp back up to where its supposed to be.

I have no Idea why this is going on, thoughts lead to the fact that there could be mash/water seeping by the seal and filling the foam core with moisture (the cooler doesn't seem to be any heaver than normal.)
Other than that I'm at a loss....maybe investing another 40 bux in a cooler.
any thoughts?
thx

 

[BrewMoreBeers]

How do you know your thermometer is correct? On my last brew I learned that my "goto" instant read thermometer was no longer good. It was off by nearly 10 degrees at mash temps. It was not consistent. In the same hot liquor it would swing between accurate and +10 when compared to two other probes.

 

[ChuckO]

Check to be certain that there isn't a leak at the fittings. A leak might allow the insulation to become saturated with liquid, which will conduct more heat than dry insulation.

 

[wilserbrewer]

Are you preheating well? I have found that you need to let the mash temp stabilize for about five minutes before it is at a constant temp. If you take the temp immediately after adding the grain your temp will be artificially high and will drop more due to the cooler grain. Somehow I feel your technique has changed rather than the cooler?