Sorry guys, been away for a while. So as it turns out, the issue went away completely once I put the bush-hog on the back, which is much heavier (2,600 lbs vs. 800 lbs). I have since switched back to the log grapple and the issue has not returned. Perhaps if I leave the light grapple on there for a while, the symptoms will develop again?
I'm still going to check the screen, etc, but it least it seems there was nothing wrong that required immediate attention.
I too remember reading that hydraulic manifold case. I'm glad that doesn't appear to be the issue as I'm long out of warranty!