These transmissions seem to need very clean fluid and filters, or you'll get the weirdest issues.
I had a very similar issue to yours about half a year ago:
Lost 4th gear....
In my case, new fluid + filter + a bottle of Trans-X (to remove varnish from the transmission) worked like a charm.
Might be worth a shot.
P.S. My old fluid was clear red and did not smell either. However, after I dropped the pan I realized the filter was in pretty bad shape. It appeared the previous owner(s) had drained and refilled the transmission (hence clean fluid), but never replaced the filter.