[Serusers] SER losing SIP registrations

Our company has a call center implementation on a client with up to 100 support personnel using X-Lite version 1050 softphones running under Windows 98. Until a few days ago, the softphones were logged in directly on our PBXs. Now, they log onto a separate SER 0.8.14 box and the PBXs forward calls to SER. That was needed to unify queue management. It works. But some softphones are being randomly kicked out of SER, it seems SER isn't receiving the refresh REGISTER messages from the softphones. The re-register timeout is set to 500 seconds on the softphones. There's a lot of "removing spare zombie" and "Binding '<user>','<url>' has expired" messages in /var/log/messages. For now, we're instructing the client to increase the timeout to 10 hours on the softphones in which the problem happens most often. I don't know if that's really the right thing to do, I think we should somehow make sure the re-registers are done in a timely fashion and retried, but I could not find ant SER configuration option related to that. What should I do? Another thing: the softphones take a horribly high amount of time to log in, but that may be because X-Lite sucks. Regards, Juan