I have a digital thermometer that I generally trust. I've had thermometer issues in the past but this one seems to be stable. That said I'll check it in ice water and boiling water the next time I set up to brew. I always seem to loose a few degrees when I pour the liquor into my mash-tun, I haven't dialed in how to adequately offset for that yet.
A few more details on the batch might be helpful: 60 minute mash, single infusion w/ mash-out, no sparge, OG: 1.055, yeast: Wyeast 1028 (London Ale). Dark grains were cold-soaked a day ahead of time so I didn't add any salts to the mash.
tomsawyer: I'm not sure what you mean by "give it plenty of time for the glucans to come out". I agree with the other posters that a shorter mash would help limit the wort fermentability, although I'm not sure how mash time would affect beta glucans. Figure 91 in Chapter 14 of Palmer's book shows the active range of beta glucanase cuts off around 120 F, so a normal mash would denature that enzyme and prevent breakdown of beta glucan during mashing. Are you suggesting that the beta glucans need time to soak out of the grain? If so would doing a "rest" between mash-out and lautering do the trick?