The network in question has just been moved to a WHM/Cpanel VPS with 512mb of ram and gets about 5K visits a day.
Shortly after the move I noticed enormous load averages (up to 30). I've tried recompiling, reconfiguring, updating everything, caching, searching forums, default theme....seemingly everything.
I mirrored the busiest site on the network (as a single WP install, not multisite) on another VPS (same host) and the load was fine (0.02).
A lot of caching and performance tuning has gotten the load averages down to about 1-2 - which is still too high, but at least the sites stay online.
Looking at the php error log I see a lot of "WordPress database error You have an error in your SQL syntax;" and out of memory errors.
Anyone have any idea what could be the cause?