I am having a problem with my 1999 Forester.  Mechanics are stumped.  It runs fine for the first 10-15 miles, and then begins a slight vibration which continues to get worse and worse.  When climbing even a slight hill, it will start vibrating violently.  The whole front of the car will start shuddering.  I've replaced the drive shaft and the top oxygen sensor, but did not resolve the problem. Fluid level in the trasmission was low but topping it off did not help.  Replaced spark plugs and wires.  Any other ideas??

Maybe a bad half shaft. I had an Audi that would drive great, but once warmed up,it would vibrate like crazy to really undriveable. Took to a shop, the next day, the test drove it, and couldn't find anything wrong. Finally, figured out, it was a bad half shaft. Might be a source of your trouble.

5spd? could be torque bind. After the car is warm, will it smoothly turn tight circles on dry pavement?

Both of the previous post ideas are good I think. Along with those ideas check the engine an transmission mounts and make sure the tires match up in tread wear.

Okay first of all is it an auto or Manuel. Second how well matched are the tires and are there any sighns of tourque bind when turning.

How long has it been since the transmission fluid has been changed. It might be a good idea to have your mechanic go with you on a 10-15 mile drive and let them see what its doing.

Check the main/center driveshaft universal joints. Expecially the one at the tailshaft of the transmission. That front universal is difficult to see when in the car, but you should be able to check it to see if it's worn.

Don't overlook the most basic possibility; a loose wheel, check the lugs. A just slightly loose or worn lug/nut combo is tighter while cold & "looser" after warm-up. Failing wheel bearing(s), or an about to binding/stuck caliper. Good luck!

Edited December 7, 201510 yr by OLsubedrivr