GM / General Motors: '92 Safari horn, doorlocks and speedometer problem. Need help please.

I started having this problem with my Safari yesterday. About a minute
after leaving my driveway the horn gives 3 or 4 short beeps on its
own. When it does this, the speedometer fluctuates up. Immediatly
following the horn acting up, the power door locks attempt to lock (if
they are unlocked then they will lock, if they are already locked then
they just click as power is applied). This will continue to repeat
every few seconds. Yesterday, when it started, I pulled over as soon
as it was safe and restarted the engine and had no problem for the
next two short trips (3 or 4 miles). Later in the day I left the house
with it and everything seemed fine. I got to the local school zone and
looked at the speedometer and it showed 80 (I was doing about 20). The
speedometer jumped all over the place and about 30 seconds later here
comes the beeping. Anyone have an idea where all these systems might
merge together and maybe short out. Since it doesnt do it all the time
I have no idea where to start looking.
I wrote this 4 days ago. Been having problems posting.
Anyway, the van hasn't acted up in the last 2 days. Any idea what
caused this would be appreciated. Thanx a lot.

you have a short some where