Hyundai 290lc-7 rpm problem

Hi all I have a Hyundai 290lc-7 Excavator that won't hold a consistant rpm on start up. It will rev up and down in rpm to as high as 1900rpm and as low as about 600 for about the first 5 minutes of running without touching anything.
I have the auto idle turned off as this doesn't work.
I have tried charging the battery but did not improve.
Any help would be appreciated.

Seeing that the engine is computer controlled.. my thoughts would be a crank shaft sensor that reads engine RPM.?? {least expensive check}
Normally located on the transmission bell housing or near the front of the engine by the harmonic balancer.
It clears up after warm up?? I may be just dirty..or what ever its reading, {flywheel}, might be dirty??
IF it was BAD, the engine wouldn't start at all..
The engine is probably a Cummins QSB 6.7L so that should get you started on finding the location of the sensor..
Good luck.

On the QSB 6.7 the crank sensor is on the left side (if facing the flywheel) of the balancer.

Its a Cummins QSB5.9-205
I haven't found the sensors yet.
I have found it has a recorded error 45

I have looked and looked and can not find a crank sensor. Ive been checking connetion and anything else I can think of with no luck still the same. It has current error codes #40 (more then 500rpm between the target speed and the actual speed)and #45 (hydraulic temp sensor open circuit or shorted to battery +) do they mean anything?

It HAS TO BE reading engine rpms from somewhere.. find THAT sensor.