The symptom you are describing could indicate one of a couple of possible problems.
The easiest to address was a firmware issue that could cause lockups on very busy networks. This was discovered shortly prior to 2.600 and fixed in time for the 2.600 release (
http://www.qsicorp.com/engfiles)
The other may require that we repair your unit. If possible, could you load 2.600 firmware into your unit(s) and see if that corrects your problem. If not, let us know and we can investigate whether they would benefit from a repair.