I checked the fluid cold on a nearly level driveway, less than 1% slope, sloping from the front to the rear, and observed that the dipstick show that there is to much fluid in the box. It was reading between the h/l indicator for the hot read. I will check it again hot when I get back home. Could an overfill situation cause this over time.
The level when cold isn't as accurate an indicator as hot, but it does seem like the trans might be overfilled. An excessively high ATF level can cause the fluid to become aerated, especially as it expands when warm. Aerated fluid won't properly transmit hydraulic pressure, leading to incorrect operation.
When you check the hot level, also note if there are bubbles in the fluid on the dipstick. By the way, it's a bit tricky to get an accurate reading of the level. I'll see if I can find an older thread where a technique was described to help with that.
EDIT: The following is from one of my previous posts.
What I'm about to describe is based on what Subaru suggests and my own experience.
The first step is to get the trans up to ''operating temperature''. How long a run that takes depends on whether the car is being driven or just idling, the ambient temperature, etc. It could be a few minutes of highway running, or 30 minutes idling.
Once the fluid is at the correct temperature, you should step the selector through the gears, and back to Park. Pull the dipstick, wipe it, and leave it out for about two minutes. That allows any fluid in the dip tube that has ''wicked'' up the dipstick to flow down, and for the fluid temperature (and excessive expansion) to normalize if the car was just driven very hard. Then insert the dipstick fully, remove it, and look at both sides of the stick -- you may find a different level indicated on each side. If there's a difference, the one indicating the lower level is probably more accurate. If you see a significant number of bubbles on the stick, the fluid has become aerated, and even if the level seems correct it may not be able to be trusted.
Edited by OB99W, 21 August 2009 - 03:24 PM.