I have a 2003 Subaru Legacy L 2.5l and while driving down the road my tach bounces up and down about 200 rpms but when at about 50-55mph my car keeps shifting up and down from overdrive and down a gear and then back up. This sometimes happens around the 30-35mph mark also. I'm not sure that this is related but at about 65mph my tach reads at about 2750 but when I set the cruise it jumps up to about 3750. There is no jump in the tach when at idle. Any thoughts, suggestions or questions? I'm hoping my tranny is not on the way out and that this could be an easy fix. Please help!

Well lets start by looking at the tranny fluid. Is it a nice pink color and smell unburnt?

 

When was the last time the fluid was changed.

 

What happens when you leave the car in D3 at the same speeds and circumstances?

 

Is this all flat ground? 

 

How many miles on the car?

Check and clean your battery terminals, also check for any broken or corroded ground wires 

when I run the car in d3 I have no flucuation in my tach. The fluid is a nice red color and smells just fine. the tach bounce and shifting issues happen on flat ground and hills. I haven't noticed it on down slopes but I usually coast those, will have to look next time I drive. I'm not sure when the fluid was last changed. My car has 163,xxx miles on it. As to wires and all, I just replaced my battery last month and all the wires look good running to the battery. I will look more into that. Any help is appreciated as my car is just sitting now and I don't want to drive it and make it worse.

does it feel like a slip or just jerkyness.

no slip felt. it just keeps popping back and forth in gear.

Next time it happens ride the brake a little bit, not to engage the brakes, but lets see if that deactivates your lockup torque converter. ALso inversely give it gas and see if it stops while accelerating