Since you have already done the 'standard' cleaning procedures, it's not likely to be crud on the pins.
Use a stong flashlight, and check where the pins sit in the connector: Are any of them out of alignment?
Are any of them bent? Patience and a small needle or stick pin can move them back into place - but be careful not to pop them out of the front.
Then, use a continuity checker (or multi-meter) and a stickpin to make sure there are no breaks in the connector. If you use an alligator clip to hold the stickpin on one end of a probe, you can make contact with the pins from the front of the connector, via the small holes at the front. Check to the appropriate pin on the back, and to the board itself. Could just be a poor/loose solder joint at the board.
If all that checks out, its not the connector.