I'd suggest that you do these in this order:
1. install litespeed tryout and see if that makes a difference.
2. check Phpmyadmin's processes view many times and see which queries give you state "Locked" and stay there over 1s. In a good server you should not see more than 5-10 processes at a time in process view. When you get locking problems you see dozens or hundreds of processes on that list with waiting state, and only few as locked.
If you see that you have locking problem, you best bet is to
a) disable all extra scripts (non-forum) and plugins and see if that helps.
b) contact Invision support and ask them to check it out, especially indexes and that forum scripts are up-to-date.
c) try other hardware. I have seen sql locks cumulating because hardware is faulty (SCSI adapter or disk). I've seen servers go bad.
d) try InnoDb, but you MUST ask Invision support about that first. Note also that InnoDB can not be backed up with mysqlhotcopy and repairing InnoDB can be a pain.
3. Stop using RAID, and use empty disk for mysql databases only.
Basically this is done in shell
a. stop mysqld service
b. move active database folder to empty drive (check permisisons and flags!)
c. edit my.cnf, add under [mysqld]
d. restart mysqld service
That database drive can be partitioned to EXT3, and you can disable access time updates (noatime) to speed it up.
I use dedicated disk for system, dedicated disk for domains, dedicated disk for databases, dedicated disk for backups.
Don't do anything unless you're sure what you do and what it does.