Jim,
You didn't say what coach / engine combo you have. In any case, do you have the economy mode selected on the transmission panel? If not, some engines / trannys will search and shift from 5 to 6 and back again at about 60 mph. This can feel like the cruise control is dropping out.
Steve
PS; you can add your coach model and engine by selecting Forum Profile and putting the info in the signature block.