Well, I went and read up on the cruise system, and while I didn't find your problem, I found some interesting stuff.

With 31's, I've had a hard time staying out of a 4-3 downshift in the hills in the 89 v6, and when it downshifts for about the thrid time, it doesn't want to upshift even after you crest the rise, unless you hit the set/coast button. I found out why it won't upshift (after about the 40th [exageration of the gross sort] time I'd set the TV cable length). There are aparently two power transistors in the o/d relay box in the ground path for the relay coil. If you get downshifts too often, the ecu cruise circuits kill the o/d for about 10-15 seconds by sending a zap to one of the power transistors in the o/d relay. I think another part of my problem is in the adjustment of the throttle cables at the cruise actuator cable drum.

From a pretty quick look at the circuits, I'd check the brake switch, which has two switches inside. One is NO and is the brake light switch. The second is NC and when interrupted by braking, kills the cruise set speed memory. Also check per Macka above at the lower column connectors and wires in the column.