when was the last time it was serviced???
The way it sounds is that the governer may be sticking or u may have a lean fuel condition,, imo i would ck all the filters and ck to make sure u have enough air flow to the unit.. i would also ck the fuel ,, u may have a bad batch,, does it do it with only one a/c running and all other ac appliances running or just with the 2 a/c units,,, ????