Subject Firebird slows down dramatically after a while

I have 1 out of 50+ Firebird installations that slows done dramatically after a time (+- 24 hours) Queries that normally takes less than 0.01 secs takes 5-8 secs). Specs of problem Server: Firebird 2.5.2 SS, 2GB, Win XP SP2 with just 1 workstation connected.

I did a backup/restore, check the hard drive and for virus, etc.  There are no long running transactions.  I have the same set-up with 20 workstations and even less ram that show no signs of the slow down even after 20 days.

Only restarting the FB server solves the problem.

Any pointers of where I must look for the problem/solution, would be appreciate.

Kind Regards