i have a 96 honda civic..when I'm in 4th gear and approaching 85-90 mph the engine cuts off (like an overspeed governor). And the speedometer jumps around real erratic at anything over 80 mph?

 

is this a statement or a question. Its sounds like your bouncing off your rev limiter. Are you at the redline when this happens?

 

Why ride around at 90 in 4th?

 

Just stating what the car is doing...looking for advice as to what the problem is. It looks like whatever is causing the speedometer to be erratic is the root of the problem, because when it bounces over 120 mph that's when the engine will cut-off.