1998 1 ton nv4500. The cruize will shut off by itself after bout 10 minutes. The light in the guage cluster is still on and you can resume, but in about 10 or so minutes it will disengage. Short in wire or vacuum?

 

Inspect the servo unit under the driver side battery. They tend to corode. May be a bad connection.
Outside of that you may have a bad connection on the vss.

The most common problem is the brake light switch needs adjustment. If the brake pedal ratles a bit while driving it activates the drop off