Engine Revving Problem

[winner]

I have developed a rather mysterious problem with my engine! I have a 1984 Airstream Excella 270 Motorhome with an LE8 484 Chev Enginer w/power heads, and a 4 BBL carburetor.
The problem develops spontaneously - As I drive down a long stretch of interstate - rather level country, constant speed, constant throttle, abt 45-60 MPH, the engine will spontaneously slowly start reving up - and will level at a higher rpm for an indefinite distance before spontaneously reving back down, and will do this sporatically over a considerable distance several times in an hour. On a recent trip from Greenville to Charlotte, it did this constantly for the entire trip both ways. No other problems evident, gas consumption remained constant and close to a normal 8 minus GPM. One Airstream dealer suggested that perhaps this involved the choke in the carb. Have had transmission checked and serviced, no problem with transmission Anyone else experiencing this aware of it?? Appreciate inputs.
Dr Joe