The table has 4 index, one from a primary key, another from a foreign key, and 2 on other columns. 

The system is a virtual machine with 8 Gb of ram, page size is 16K and page buffers 512. The IO sub system is a Dell DAS, with 16 SAS 15K disks in raid 10. Our sysadmin ran a test on the disk, and the result was 7500 IOPS. The filesystem is EXT4 mounted without barrier, and using LVM volumes. I hope this is meaningful to you, if not, I will ask the sysadmin again.

I was able to reproduce the problem on our production server witch is a Virtual windows server with 16 cores and 128 Gb of Ram. The IO system is a fiber channel SAN, no more than 6 months old.

I have also reproduced the problem on a pre prod environment(Virtual windows server). However i was unable to reproduce the problem on my laptop. 

Could this be a issue with running Firebird on virtual servers? The production database server was moved to a virtual server about 6 months ago, and our tests showed a 30-50% performance increase on all our bulk inserts/updates and general program/website performance. However we did not test how deletes performed.