Okay, i'll describe this the best i can and see if anyone has had a similar problem.

I went to the movies tonight, no problems on the way there, watch movie, leave. When i get on the highway i notice something odd. The speedometer jumped. I was doing about 70 mph and speedo jumps to 85 then back...couple seconds later it does it again and this time i notice that check gauges light blinks with speedo jump...does it again and i further notice it appears headlights dim with speedo jump. Starting to get a little worried. Hard to drive 15 miles down highway with no lights. But after doing it 4 or 5 more sporadic times, it doesn't do it again the rest of the way home. Any ideas? I would assume a loose wire, maybe to the transmission for speedo but why the dimming of the lights. Is it maybe a sign of something more serious? Any ideas or help appreciated.


Edit:
I did look at all gauges and other than the speedo jump nothing else was out of the ordinary.

 

I would assume the problem is broader in scope than just with the speedo. For example the grounds could be loose or corroded causing a drop in voltage system wide. One ground point in particular always seems to go un-noticed until it is bad. That is the ground strap from the frame to the engine. The other important ground point is right above the starter. There's one more important ground near the ICM.

I would advise you to check the grounds, the positive cluster, the connections at the alternator and the battery. Remove them, clean them shiney and retighten them. After you get that, put a meter across the battery while it's running and confirm you have about 14 volts DC.