E450 chassis AC and vent cutting out

[frogsandhogs]

We have a 2006 Lexington 283GTS on a Ford E450 chassis. Issue with the cab blower cutting in and out. The fan runs fine when on the heat setting. When the setting is vent or AC, it blows great, then suddenly stops. A few miles down the road it kicks back in and will run for maybe 15-30 minutes, then stops again. After 3 days at the Ford dealer, they tell us they can't find anything. A previous mechanic replaced the blower resister and that did not fix. Has anyone else had this problem? We can't be the only ones with this issue