Hi Jackie, very unlikely to have anything to do with the height of the fuel tank unless you have altered it recently. It is also very unlikely to be the governor mechanism as it is very robust and good for many thousands of hours.
I, like Stef, think it is a good idea to start with the simple and obvious and work up. If you can`t change the air filter, remove it all together and see if that improves things. (Please don`t do this in a dust storm).
It is interested that you can bring the engine back up to 1500 RPM manually. This to me suggests that the governor may not be operating the racks on the fuel pumps as quickly as it should or that you have some dirt in the fuel injector nozzles, either one of which will reduce engine HP, this would normally be compensated for by the governor opening the racks on the fuel injector pumps further. Did this problem come on suddenly or has it been a gradual thing over a period of months?
Does the engine produce a lot of blue, black or white smoke at any point while running?
Bob