I'm only throwing this out as it has solved a similar issue with
us. A little background first before you dismiss it. We've worked
with Interbase since version 5.5 and Firebird since the beginning.
I am very aware of the default CpuAffinityMask setting of 1 and what
it does.

We found that specifically on Windows 2003 Servers with multiple
CPUs. We tried turning hyper-threading off and setting
CpuAffinityMask to 1 and the problems presisted. Setting up people
with Classic server is not a real option for us due to logistical
reasons. As something to try we set CpuAffinityMask to 3 in the
firebird.conf file (here's the exact line).

CpuAffinityMask = 3

This worked for us and continues to work at many installations. I'm
not saying that it will work for you but I think it's worth a try.
Again this problem was specific to us on the exact OS and similar
hardware this message was posted for. Before you just dismiss it,
please try it and report back your results. I should add that we
are using Firebird 1.5.2 on all Windows operating systems. Since we
discovered this setting works on the specific Windows 2003 OS we've
tried it with our software on all Windows OS with various hardware
configurations and have not found a problem with it.

Regards,
Steve