Welcome to red, however will need serial of combine to have a clue on what to try on your machine. They made some running changes on 1660 and year is not close enough.
Your 1660 could have a filter problem . If the Hyd oil has not been changed for a long time or since new , this could be your problem. (oil and filter)
The feeder, steering and unloader auger all run off the small aux. pump on the left side back by the engine, also some have seperate relief for that pump, maybe possible pump or relief problems. But the higher RPM,s is noramal on them for hydraulic performane.
First of all you need to know what your header lift pressure is to determine if there is a problem or if adjustment is neccessary. You will probably want a mimnimum of 2200 psi. Also as RPM has said there are some changes made to the later 1660's. If the electo-hydraulic valve stack just outside the cab door is a horizontal type arrangement there will be a header lift rate adjustment on top of the valve stack (as well as a port to pressure test from). Close (turn in) the adjustment to increase the header raise rate. Also have this valve closed before you do a pressure check. Even with everything set correctly you will still have to speed up the engine some to lift the heavy headers. The auxilliary hyd pump is a gear type and relies on engine speed to create more flow.