Slow Cruise

My 89 XJ with cruise only sets to speeds below 60mph. It works fine it seems as long as I do not want to travel over 60. If set at say 70mph the cruise will not take contol until the jeep has slowed to about 50 then kicks in and accelarates to 60 and thats it. Anybody got a suggestion? Thanks!

Check all vaccum lines for the cruise assembly...also check the vaccum resovoir mounted on the inside of passanger side of the front bumber.