30 second freezes are usually caused by something trying to access a network drive which is inaccessible (as in no longer on the network, not access permissions). Could something like that be happening?
Other possible causes of temporary freezes are discussed in these two guides, but if they're usually 30 (more or less exactly) seconds long then that's the most likely cause.
Making some manually-generated crashdumps during a freeze can reveal the cause (you'll need to link your account for us to look at the dumps, please):
Process Monitor can also help track down the cause, but may be difficult in this case as it would need to be logging before the problem begins, which is a problem if you can't predict when it happens. (It can use a lot of memory if left logging all day, and impact performance.)