EvilOptic may well be right. What are the exact errros?. (look in the log file).
We seem to be having occasional problems with both Arcserve 7 (on standalone servers) and with the Push Agent on production servers. I am convinced that some of the problems related to 100mb/Full Duplex but am having problems convincing the rest of the team. A lot of cash has been spent upgrading from 10mb/HD!!.
There are some issues with various sorts of LAN adapters running Full Duplex (E100B - ours! , CE100B - ours again!). This is an old issue though. Try to fix the port speed on switches and at the card driver (INETCFG->BOARDS (board settings). Look out for, and be wary of, any values set to "0" (Auto-negotiate) this doesn't always work very well. Best to set to known values at both ends.
Personally I find 10mb/HD with push agent to be fine but we need to wring every last ounce of speed as we do full backups (about 20-30Gb) each night.
Regards
Mark