[imp] Vanishing filter rules

[Myke Place]

Recently, we have starting seeing a disturbing trend. Users with large 
filter lists are seeing them vanish and they have to be restored and 
imported back into the MySQL DB from backup. 

Is there a limit to the maximum number of filter rules that can be
Horde/IMP can handle? Has anybody else seen this problem or is it a known
bug? Does anybody have any troubleshooting suggestions? I'm not really too
keen on turning on MySQL debugging, but I will if it become necessary
though I thought it would be prudent to check here first.

TIA for any suggestions. Relevent version info follows:

# Horde: 2.2.1
# IMP: 3.2.1
# PHP Version: 4.2.3
(I'd be happy to post any other info of course)

-mp