Sudden Transmission Flaring 91 300E

[JohnM.]

The transmission in my 300E (190,000K) just started to flare between 2-3. Barely noticeable if your driving it slow, but very noticeable when your flooring it. I have driven it 2,000 miles in the last 2 weeks. Does not flare at all on downshifts (even hard ones), just the upshifts.

Just did a fluid and filter a few months back.

K1? Vacuum Modulator? Where to look?