Properly using a Meat Thermometer
I'm a little confused about its behaviour. Its a Polder in-oven thermometer (not instant read) that has a digital reader that sits outside the oven, attached to a probe.
I inserted it into the center of the boneless roast without touching the bottom of the pan and placed the roast in the oven. Once the roast reached 155 I took it out and re-inserted the probe to a different central location to confirm the temperature. Within a couple of minutes it dropped to 110 and was not going back up, I gave it about 5 mins on the counter top as I know this thermometer is not instant and needs time, but the temp. kept dropping. I put it back into the oven, but it took about half an hour to reach 155 again, and of course the beef was overcooked.
I now know that I should have of trusted the reading the first time, but is it strange that the thermometer behaved that way, or am I mis-understanding how non-instant thermometers work?
Any chance you got your probe wire wet when you repositioned it? That'll screw up readings in a New York minute.
I cook beef to med-rare at 135 degrees or so. I think the 155 degrees is probably too high - that may be your problem. I'd expect beef cooked to that high of a temperature to be pretty 'gray' all the way through.