Well I've fitted a bigger heatsink
(here) and the temperature of the regulator case stabilises at about 65 deg C compared to 70 deg C for the previous heatsink. When the display is off the temperature drops - running the clock program from the manual the temperature stabilises at about 62 deg C. I'm a lot happier with this as the previous heatsink never seemed to stabilise, and I was worried it would creep too high. 65 degrees is pretty hot, but not unusual for this type of regulator. This larger heatsink means the INS8154 RAM/IO chip probably won't squeeze into its socket, but I'll shift things around when I update the PCB to make room for an even larger heatsink that will cope with the added current load of the RAM/IO and the optional memory.
I think the reset problem I'm seeing is something to do with the switching power supply I'm using, because if you leave the capacitor to discharge or short it before plugging in the MK14 resets properly every time.
I've been having fun playing around with the MK14, trying to get my head around to the relative addressing scheme it uses and the minimalist instruction set, and I'm so happy to have a working system at last!!