[vanir]

This sounds hard to figure without more information, mechanicals just have so many moving parts it can be a trick to pin any issue down immediately.

Tell us what your idle rpm is in gear, stationary.

As mentioned the tranny soaks up some rpm so even if it sounds okay in neutral, it might be idling too low in gear to run the generator or keep oil pressure up.
You're supposed to set idle on autos in gear. No, don't put it in gear and start playing under the hood, you keep it in neutral, set the idle and then put it in gear with your foot firmly on the brakes. Check the rpm in gear is the factory setting (should be around 700-900rpm depending on the engine type, but look this up).
Too low? Put it back in neutral and up the idler on the carb. Too high, set it down. Keep adjusting until it idles properly in gear, then tell us what it's doing in neutral.
It should jump to about 1200rpm in neutral, again depending on the engine type (this information should also be in a manual for the model).

ie. if its revving its ring off when you put it back in neutral just to keep it idling when you put it in gear, you've got a problem. Which is probably just something like running waay lean and too much retard, a tiny vacuum leak or that sort of thing.