I do recall a problem with my ACE 29.2 a few years ago, I was having the same problem, only when the outside air temp was above approx 75-80 deg while driving down the highway, the Genny would shut down. On that particular model 29.2, the Genny was installed on the drivers side, behind the left front wheel. It was fairly close to the engine compartment.
After some research I learned that the gen was too close to the engine, as you drove down the road and heat was carried away from the engine compartment, that heat traveled right past the running gen, which would add to the heat already produced by the gen and create a hot zone, which would overheat the gen and it would shut down to protect itself from damage.
Thor had apparently come up with some type of heat shroud modification to increase the airflow and prevent the stagnation of heat in that area, I don't remember the specifics, you can do a search of these forums and find the thread. This was about 3-4 years ago!
It appears on the larger class A's, Thor has consistently put the Genny on the drivers side towards the rear of the coach, away from the coach engine, I haven't had the problem with the Miramar so far.
Hope this helps!