TBH not something I'd mess with since it would appear to be in relation to the ATM configuration commands and way over my head - sorry.
The only ATM config that we normally have to set is (for BT) is VPI = 0 VCI = 38
Theres a bit more info in the 3com CLI manual
http://support.3com.com/infodeli/tools/routers/3ComComRef.pdfParameter
output-pcr: Peak rate of ATM cell output in Kbit/s. For the value ranges of this parameter, see Value ranges of output-pcr.
output-scr: Sustainable rate of ATM cell output in Kbps. Its value ranges are the same as those of output-pcr.
output-mbs: Maximum burst size of ATM cell output, i.e., the maximum cache size of ATM cell output at the interface in cell number.
See ATM configuration Page 343 onwards.
Theres a discussion on files stopping at 99%
here which may give some clues where to look..
Something else I would also check out is the
maximum no of IP sessions which has been known to give some people problems in the past.
Particularly if you are using applications which utilise multiple sessions such as p2p.