Go to
Kubotabooks.com/ Tractor parts manuals and download the parts manual too.
The RPM's changing at high RPM's would probably be a fuel or Air delivery problem. Could be as simple as a plugged vent on the fuel tank cap, clogged air filter, to as complicated at a failing injection pump.
I would start at the fuel cap and go threw every piece of the fuel system, and check/replace the air filter.
Clean the vent on the cap, check the fuel lines (soft or cracked lines are bad too), pull the filter clean bowl and replace filter, check fuel flow in and out of the filter (pull the line off that goes to the lift pump), then pull the line from the lift pump to the injection pump, it should pulse and move a small amount of fuel while cranking, then pull the injector lines off the top of the injector pump and you again should get fuel to pulse while cranking, last pull the injectors disassemble and clean.
NOTE: You will need to bled air out of the fuel system as you go, so when you pull parts it will take a little cranking to purge the air out of the lines and parts