They're replacing the cluster now, but had to order the part (despite my having a week lead time prior to my appointment and a correct diagnostic).

Yeah, the temp gauge could be a thermostat issue, but it has held steady at the new point for more than 3 months now, which doesn't sound like any bad thermostat I've ever encountered. The fuel gauge, however, was the problem precipitating the repair (and I wouldn't even know if my speedo or tach were off).