Check where the tubes are welded to the head "frame". You may find them cracked. Mine did, after aboot 6 yrs, and were even more oval'ed out!! Cequent replaced the entire hitch with a newer model. There were also cracks in the upper welds, though not as severe. It looks to me that there has been lube in the past as you can see the dirt sticking to the area around the release clips.
I posted about this back in Oct 2013, you may be able to find the posts, but I don't think the pics will show up.
If you find cracks, you might consider giving Cequent a call, and letting them know what is happening. Even if they don't do any replacement, they will have another report of a problem for their records.