[sluggo]

Not a Kohler expert but you typically need to determine of it is a mechanical issue (engine overheating, fuel starvation, etc) or a control issue (bad relay, control board, etc). For starters would wait for a shutdown then check the state of the sensors (coolant temp, oil p, exhaust temp, etc) to see if one of those are tripping a shutdown sequence. otherwise as Capt Brett indicated you can get real deep trying to troubleshoot control issues and or intermittent problems like an electric fuel pump going bad. A service manual and DVM are pretty important for troubleshooting. If you turn to a professional make sure you find someone who has experience with generator controls.

Thanks! I have changed out most of the sensors, this electric fuel pump seems to come up quite often as a possible problem, maybe I'll just swap it out just to see what happens.  I figure it cant hurt, and besides its a 2006 so its 12 yrs old.

FYI  I always like to double back, and if I found a solution, to let the next guy know.  We think we have it fixed, he changed all the stuff we could mechanically, fuel pumps, sensors, fuel filters etc etc.  Finally we switched out the electronic board, and I think we have it fixed!! knock on wood!!  It ran for 4 hrs yesterday, with out missing a beat, and put out good, stable power!  I think the board was around $140.

Well to triple back on this, it looks like its not fixed.  So frustrating, I don't know what to do next.  As I mentioned, when I took it to its winter home, it ran fine.  When I brought it back to its summer home, it ran fine.  But in review, what kind of load was on it?  I didn't run the A/C I know going down , and I cant remember on the way back, plus it was relatively cool outside.  Does the outside temperature have some effect on all of this? IDK.
Now that it has gotten hot outside (90+) and the river water temp has risen into the mid 70's, it is only running an hour and shuts down.  I suspect a heat issue, but I cant find where.  We blead the lines and did get some air out of them, but it still only ran 1 hr.
The bigger problem is, I don't have a certified marine A/C tech anywhere near by!!  I am not sure what to do next. 
Any thoughts??