(by Ethan Anderson)
(The information below applies mostly to Motion firmware 20 & lower)
The factory settings cause indefinite temperature swings of ±5ºC with a period of 60-80 seconds. M303 (PID auto-tune) causes the printer to crash/reboot in firmware versions preceding 18.37. In 18.37 and later firmwares, M303 works, but the results are on par with the PID settings from the factory. The stored value of the I parameter is 1000th of what it actually uses and autotunes to, therefor after M301 autotune you will need to manually reset the M301 I value to 1/1000th of what the autotune reports.
Solved by starting from square one with P, I, and D values at zero. Raised P until the temperature stabilized, then raised I to properly offset the stabilized temperature to match the requested temperature, then experimented with D to minimize the time taken to stabilize. The factory settings are: M301 P63.00 I2.25 D440.00 My new settings are: M301 P20 I0.02 D250 The full M301 has P, I, D, C, and L components. I did not change C or L, as I'm not entirely sure what they're for. I couldn't find any reference to them, but I think they may be the same as T and S (seen here http://reprap.org/wiki/G-code#M301:_Set_PID_parameters). If anyone knows I'd love to hear it!