WARNING: invalid client??

HI

I have one card with about 9 users on it.
It was working fine for about 6 months but the last few days getting this error.
It can happen at any time.
Sometimes it can work for 15 minutes and sometimes only a few few minutes.

Anyone help?

2012/12/20 21:33:39 A143210 c WARNING: invalid client
2012/12/20 21:33:39 A143210 c johnwork disconnected from xx.xx.xx.xx
2012/12/20 21:33:40 A15D220 c WARNING: invalid client
2012/12/20 21:33:40 A15D220 c patworker disconnected from xx.xx.xx.xx
2012/12/20 21:33:56 A144E70 c WARNING: invalid client
2012/12/20 21:33:56 A144E70 c patultimo1 disconnected from xx.xx.xx.xx
2012/12/20 21:34:03 A15B9C8 c WARNING: invalid client
2012/12/20 21:34:03 A15B9C8 c joelarchville disconnected from xx.xx.xx.xx
2012/12/20 21:34:04 A155868 c WARNING: invalid client
2012/12/20 21:34:04 A155868 c bernard disconnected from xx.xx.xx.xx
2012/12/20 21:34:07 A15A170 c WARNING: invalid client
2012/12/20 21:34:07 A15A170 c joebed disconnected from xx.xx.xx.xx
2012/12/20 21:34:08 A153618 c WARNING: invalid client
2012/12/20 21:34:08 A153618 c paul disconnected from xx.xx.xx.xx
2012/12/20 21:34:12 A164BD8 c WARNING: invalid client
2012/12/20 21:34:12 A164BD8 c pat disconnected from xx.xx.xx.xx

all these names in for the last few months

All these users added about 6 months ago

I will put up config later when i get home from work