Hardware:
RADDS 1.5 w/Arduino Due r3
AD8495 Thermocouple Amp w/Type K Thermocouple on Hotend
SD6128 Stepper Drivers
Printer has been running great for the past year or two, then about 2 weeks ago I started having the following issue. When I power up the printer, the temps for both the hotend and bed show normal ambient temps. I kick off the pre-heat cycle and both the hotend and bed start heating up, however after about a second or two, the hotend will suddenly display 409° C. It will sit that way for a minute then throw a thermal runaway error. I can hit the reset button a few times and then it will start working properly. The first thing I did was to replace both the thermocouple and the amp with known working parts. This seemed to have fixed it for a few days, but then it started doing this again.
The other thing that I noticed is that sometimes after kicking off the pre-heat cycle, it will display 409° C for a few seconds, then it will display the proper temp of the hotend for a second. Then it will once again show 409°. It seems like if I keep resetting it, it will eventually start working. Once it is up to temp, I can run multiple prints back to back with no problem at all.
Anyone have any idea what could be causing this? Thanks in advance.