If the temp reading is correct and stable, but the depth reading is spotty, it might be a single bad pin or conductor. I'm not sure but I think there is a very small gauge coax that carries the signal data from the transducer. If the cable gets kinked it can break or damage the small gauge internal conductors. Recheck the pins and sockets. Sometimes even a little corrosion can cause problems. The signal voltages are pretty small. I always put some dielectric grease around the connector to keep water out. Sometimes I would accidentally get a blob on the connector pins and I would get some very erratic readings. Try cleaning the contacts again with alcohol or contact cleaner. If you have some hard corrosion or pitting you can sometimes clean it up with a dental pick. Just be careful; those pins are fragile.
You might check around to see if somebody has a compatible transducer that you could plug into your head unit for troubleshooting. That would give you a good idea if it is the transducer cable or the head unit.
Check the Humminbird site for compatibility:
http://store.humminbird.com/products/314027/XNT_9_20_T