I recently got a 7.5KW marine diesel Onan generator for free. He said it hasn't been run in a while and the voltage regulator was bad(wired for 220 but said it puts out 300v).

Anyway brought it home, stuck the fuel lines in a gallon of diesel and it fired up fairly easy. I turned it on and off a a few times and it worked great. I then took the hoses out of the gas can and put the can away. I came back to it a few hours later and it took alot of cranking to start(figured it was because the fuel lines were empty). After it did start the governor hunts/surges. The engine will start to slow way down and the governor will slam on the throttle, then back off, then back one, instead of staying at a constant RPM.
Any idea what might have happened? Why would it work fine one minute then after sitting for a couple hours suddenly develop this problem?