"have obviously been setup and working correctly."
Just because it worked doesn't mean it was setup correctly. My friend had a house with 2 1400W BaseBoard Heaters and one 1000W BBH on a 20A circuit, how did that work (31.6A required)? The previous owner had wired 2 of the heaters in series and one in parallel. So the 1000W heater was running as if it was 1000W but the 2 1400Ws were running as if they were 700W heaters. They were both actually a little high resistance over spec so they were drawing just under rated current. Did I mention this was on 14-2 wiring? .
Disconnect the thermostat and measure the lines in there, maybe that's your issue. The thermo could have went bad when the last unit went bad as well, if it's the old school type disconnect the wires and check the resistance across the switch when the temp is set to 50 and to 80 (assuming your house is 70 degrees inside), it should measure ~0 when set to 50 and >1Meg when set to 80.
10/1/11: Bought the perfect house;11/1/12: 10k & 200 hours into the perfect house; Floors are done and cutting up the cement for the new bathroom that wasn't actually plumbed like the seller said it was; 8/1/13: 30k & 400 hours into the perfect house that only needed new carpet and a coat of paint...
12/1/13: 35k & 500 hours I hope it ends soon
02/1/14: 37k & 550 hours and the inside may be finished soon.