> At some point in january (or close by) we changed some settings regarding GC.

Which settings ?

> This was done because we had problems with our Firebird 2.1.3 freezing every other day.
>
> The problem actually went away. For 5-6 weeks. The problem is still there, but not so often anymore. But as before everything freezes, cpu is 0% and only around half of the installed 12 Gb RAM is in use.
> No one can connect (not even SYSDBA in any way), those connected can not do anyting.

Could you provide coredump of such freezed process(es) ?
And copy of lock-table.

> Eventhough the problem has been less than before, its still there.
> It can happen with few connections, a lot of connections, no special attivity or a lot of attivity.

Are you use EVENTs ?

> As of now we would like to try a snapshot build of Firebird 2.1.4. Just to do something.
> If we install Firebird 2.1.4, and find troubles of other kind, can we then just downgrade to Firebird 2.1.3? Or upgrade to another (newer) snapshot if some is available?

Yes, of course.

Regards,
Vlad