Since PhotoMechanic is just a company trying to create a non-free version of Adobe Bridge, it might be the issue in this scenario. Not saying to get rid of it or anything. Just wondering if you have tried to go through Bridge instead as a test to see if you have the same issue? It's possible that the version of PhotoMechanic you are using is not fully up to date with the latest version of photoshop and camera raw, which might be throwing things off. It's worth a shot to find out. And since PhotoMechanic and Bridge are virtually the same thing and functions almost exactly the same as it pertains to speed and all that, it might give some insight as to what is happening with your workflow at the moment.
Thanks Daniel - going to give Bridge a go and see if the issue is replicated. Have to say though that PhotoMechanic is so much more than a non-free Bridge - the captioning and ftp transfer function make it almost an essential part of the workflow in my business. I'll not be happy if that's the root of the problem!!