When I’ve driving my car for more than 2 hours, the tach jumps from 3K to 2K, losing power. I have to pull over and wait for 30 minutes. Then it’s fine the rest of the trip, which is 4-5 hours. The electrical is fine and the car hasn’t over heated. The dealer has seen the car 3 times and can’t find anything wrong. When the car had 100 miles on it, the fuel pump had to be replaced but that’s the only thing that’s gone wrong other than now.
The problem is not with your tach. The car is losing power. Why is anyone’s guess. Does the “check engine” light come on? Does it run roughly?