Under "Optional Features":
"Common Page 1" tab, make sure your mic sense is set to "normal".
"Common Page 3" tab, make sure your PTT ID settings are correct (no long pause times, etc).
Under "Extended Function":
Make sure you don't have an option board selected, unless you really do have it installed.
Make sure "Universal Connector Pin 11" is set to Always 5 v (high).
If that doesn't work, it may be a damaged connector on the radio, flex cable, etc. Was this a new radio that used to work, or something you bought used?