Lockups!

We've recently been enduring a spate of lockups. Scenario...The sqlturbo process starts up and goes NOWHERE. any user on the database gets complete freeze. The usual check of tbstat -l shows that we have lots of log space, but that a checkpoint is required. An attempt at tbmode -c goes off for a while, and then returns the prompt, having NOT done a checkpoint, but certainly NOT told us about any error. "Force checkpoint" from the online menu locks tbmonitor menu up solid. We can take the database offline with an "Immediate shutdown", but this is not really an acceptable solution. Anyone have any ideas why this is happening? Do you need more info? If so, I can gather at the next occurence. Brian P. Hampson