That's worrisome.
Could be the ignition switch is sticking keeping the glow on.
Quick, simple cheap 'maybe' solution is to squirt WD40 into the key slot.
That will liquify and dry lubes that may be causing your problem. Nothing to loose.
Those 4200 glow plugs are turned on when the ignition is turned to the run position,
then a timer relay under the dash takes over, like Piloon said, your switch may be re-cycling the relay,
or it could be the relay itself or a wiring issue.