This may sound crazy but check your stop/tail light grounds. Try taking both tail light bulbs out and try the cruise.
If the light isn't grounded properly the tail light can backfeed to the stop light circuit which will cancel the cruise.
You will usually see a problem with one of the tail lights being brighter or dimmer than the other.
Look closely at the bulbs when the tail lights are on. If the shorter of the two filiments is lit at all that is your problem.
The grounding or the bulb socket is usually the problem.