need codes for 99 durango

We are having a stalling problem with our 99 Durango and getting codes "no bus", "920", "921" and "999" but cant find any information on any 900 series codes. After reading some of the other forums, it seems that our PCM/ECM needs replacement but we are trying to rule out any other causes first. We tried to reset the PCM and arent sure if we didnt do it correctly or if the PCM is shot. Any suggestions would be greatly appreciated.

update - my dealer didn't know either, but when I picked it up (after they replaced the pcm) they said they were noncommunication codes, pcm to ecm, pcm to tcm, & pcm to ?