The first thing I'd check is the oil level. If it's just a "smidge" low, it may not be low enough to shut down when parked and level, but a "sloshing" (sort of) of the oil when underway may "trip" the low oil shutdown.
The Microlite 2800, IIRC, is the ONLY Onan genset that has a unique oil level requirement. IIRC, it should be to the "top" of the dipstick/fill hole - by "top" I mean if you added more oil, it would simply drain out of the dipstick hole.
Some folks choose the put the dipstick in the fill hole - not screw it in, just "lay" it in the hole - to check the oil level.
The first way may result in the oil level being a skosh high, but no harm IMHO, so that's what I've used for ten years.
Cheers.