I was having the same issues with mine and followed Walter @ March Pumps suggestions that he wrote re: a march pump that was doing the same thing. I have not had any problems since.
"More then likely what's happening with yours is that you are getting a sugar buildup on the shaft the impeller...
I ended up going to speedy metal and getting some sheet SS that I lay on top, however the more I brew with the penrose kettles the more I find I dont use lids at all. Here is a pic of my setup with the lids on top.
I have to say that I love my Kettles as well as my brewhemoth, So much so I...
Just started my first lager yesterday with my brewhemoth. I have a similar setup where I am recirculating glycol out of my kegerator freezer. Temps are being controlled by a crestron automation system.