I don't have any help to add to what you already have been given that is specific to your problem, but just a couple of pieces of general advice:
First, Do not mess with any of the stops on the injection pump. Adjusting these is a complex and fussy job, and unless they have obviously moved, they are not going to be the source of the issue.
Second, The throttle cable should have enough "throw" that the throttle arm on the injection pump comes to rest on its stop when fully pulled to "slow". If it does not, adjusting the throttle cable is certainly something easy and low risk.
Last, if the engine mounts are "old" (15-20 years?) they probably are due for replacement anyway. The do wear out and replacing them is a scheduled maintenance item.
Good luck with sorting out your issue!
Hollywood, FL, USA