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I have a 94 and don't know how different an 86 is. Usually when my cruise quits, especially if I have been doing some maintenance on the passenger side of the engine bay, it is usually one of the vacuum lines attached to the air conditioner against the firewall. My fumbling around will often pull the plug loose even when that is not what I am working on, usually changing spark plugs is when I have this problem. Be sure to feel the vacuum plugs to make sure they really are plugged in. On my 94 the plug can pull off and then settle into place and look like it is plugged in when it isn't. On mine I think it has 3 lines in a triangular plu. It will pull off and settle back onto 2 vacuum ports and looks like it is OK. This will usually trip the CEL (check engine light) after a few miles. Don't know if 86 has this.
Another spot that can kill the cruise is the ABS sensor on top of the differential. If it goes bad it will usually trip the ABS light, kill the cruise and make the speedo go flakey. Don't know if the 86 has ABD and whether it is attached to the speedo and computer.
Like I said, on an 86 I don't know if any of this is even there.