My production server is running 4.0.20-MAX so our versions are close. I'm not running phpBB but another one with an even WORSE schema. My db has problems due to the schema and code. I'm working with the developers to remove bad queries (LOVE slow query log!).

That said -- the db NEVER just shuts down except for problems relating to ulimits (the ulimits for mysql were not bumped up, I asked the sys admin and they assured me they were, I bumped up the table_cache and brought down the mysql server. I asked for specific cmds run as mysql and found that ulimits for mysql were not set just the system. Today I start upping my table_cache. We'll see if I crash the server again!).

I'm interested to hear if phpBB is triggering some bug in MySQL. I'm surprised your query cache isn't being used more?? My forums software has more blobs in almost ALL the tables (my programmers thought didn't understand the ramification of this act) than phpBB. We've been changing many of the text datatypes to varchars and enums. And still, with all those blobs, more of my queries are being cached. It takes about 24-72 hours for my query_cache to get into the 80-90% used range. The server still prunes too much and writes too many temp tables to disk.

I'm going to start the process of moving to 4.1 from 4.0. My programmers are afraid the timestamp format will break everything as well as the character set stuff. we'll see. Have you thought of moving to 4.1.?