How does the data get from the radio to the TNC? If it is through the speaker output you may need to adjust the volume setting on the radio. It may be too high or too low.
Do you have a buddy with APRS? If so, have him drive around your neighborhood and adjust your settings until you can receive data correctly.
He said he's using a TS2000. The TNC is internal to the radio.
Perhaps the TNC is set for 9600 and not 1200 for VHF? I have a TS2000 but don't use the TNC portion of it so can't offer much advice. Nothing around here to talk to unless I put up the other end of the link too