Constant disk activity noise: "dtrace: error on enabled probe"

Recently my iMac (Late 2009, 27", 2.66GHz Quad-core i5, 8GB memory, 1TB HD) has been very slow. Launching applications, opening files, saving files all seem to be slow and I see the "spinning beach ball" quite frequently. The ability of my iMac to do any work has been significantly adversely affected.

Opening the Terminal application and typing "sudo opensnoop" reveals a constant rapid stream of this:

In case it's not clear in the inserted image, most of the lines read like this:

dtrace: error on enabled probe ID 5 (ID 152: syscall::open:return): invalid user access in action #11 at DIF offset 24

Anyone have any idea what this is? This constant error reporting appears to be the culprit.

Thanks for any advice.