Marlin - Heated Bed not maintaining set temp

I have just swapped over to Marlin on my Rumba board, from Repetier due to skipping issues on the y-axis that I could not solve. With Marlin, the random skipping completely disappeared.

My bed is running on PID and successfully autotuned. It will get to the set temp (120') but once the print begins, it will quite rapidly fall to about 105' then very very slowly make its way back up.



I had the same problem using bang-bang on the bed. This really ruins everything I want to print as the corners lift dramatically.

Does anyone have any idea why this would be?

Here are my settings in Marlin v1.0.2:

#define PIDTEMPBED
//
#define BED_LIMIT_SWITCHING

// This sets the max power delivered to the bed, and replaces the HEATER_BED_DUTY_CYCLE_DIVIDER option.
// all forms of bed control obey this (PID, bang-bang, bang-bang with hysteresis)
// setting this to anything other than 255 enables a form of PWM to the bed just like HEATER_BED_DUTY_CYCLE_DIVIDER did,
// so you shouldn't use it unless you are OK with PWM on your bed. (see the comment on enabling PIDTEMPBED)
#define MAX_BED_POWER 255 // limits duty cycle to bed; 255=full current

#ifdef PIDTEMPBED
//Mk2 12v HBP with 1/8" glass
//from FOPDT model - kp=.39 Tp=405 Tdead=66, Tc set to 79.2, aggressive factor of .15 (vs .1, 1, 10)
#define DEFAULT_bedKp 123.52
#define DEFAULT_bedKi 13.34
#define DEFAULT_bedKd 285.84

//120v 250W silicone heater into 4mm borosilicate (MendelMax 1.5+)
//from pidautotune
// #define DEFAULT_bedKp 97.1
// #define DEFAULT_bedKi 1.41
// #define DEFAULT_bedKd 1675.16

// FIND YOUR OWN: "M303 E-1 C8 S90" to run autotune on the bed at 90 degreesC for 8 cycles.
#endif // PIDTEMPBED

After about 40mins of printing, it made it back to 120 deg and seems to be doing what it is suppose to, but the earlier 15 deg. drop has had bad effects on the print.