It really seems to be a problem related to the GC options mentioned here. However there seems to be more to it. With the rather aggressive setting of only 6 seconds between full GCs we were able to reproduce the problem, however only on a very specific combination of hardware, OS and Java VM.
Running on RedHat 9 with Sun's Java VM 1.5.0_09 we could only see the problem on one machine that appears to be the same as all the others we use. However as this very same machine shows no problems at all when running our application with Java 1.4.2_08 or 1.6.0RC - even with the 6 seconds interval - I do not believe in faulty hardware.
I removed the machine's hard drive and put it into an identical system (as far as we can see and are told by its manufacturer). In that box (and two more I tried) I cannot get any crash with either Java 1.4, 1.5 or 1.6. Strangely enough even the drive from one of those other machines mounted into the problematic box showed no problems. We really do not have any clue here.
Nevertheless I have now moved everything back to where it was and have increased the GC interval to 4 hours. So far I have not seen any more crashes. The system will be running till January 3rd. If there were no crashes between now and then we will consider this parameter as a solution to our immediate problem, even though the underlying cause may never be found...