On a rare occasion I've noticed my car won't shift into fourth gear. When it happens its in the am only and cold out, if I can catch a red light long enough I can shut the car off and restart it and the problem goes away for as much as weeks to months before it happens again. I can't recall it happening any more often then maybe twice or at most three times a week and not 2 days in a row

Since I work next to a shop I asked what they thought it could be and they scanned it for codes saved in memory (check engine light only comes on when its happening) and they came up with: valve body dirty/restriced, 4th gear clutch pack worn out or damaged seals.

The trans fluid is clean but due to be changed again. Any ideas which problem this may be or some other problem?