I've got a 1994 kobelco sk220 mark lll s/n LLU0819 with the Mitsubishi 6 cylinder and it won't throttle up or down with the throttle controller, it will run at what ever rpm you desire if you turn the switch off and physically move the linkage on the injection pump with your hand then crank it back up.
It sounds like u know what the problem is.. Have u tried going thru the "calibration".?? Look up "throttle calibration".. its been discussed here before & u might find it doing a search on this forum.? If not, just googlewho it..