Hi. sorry this response might be a bit late for you but I've only just joined this forum and it might help others that read the thread.
I have done two of these engines in the last 3 months. it is definitely the rings. They stick in the pistons.Try pouring coke (yes cola) into the plug holes and leave it overnight first then an oil and filter change for obvious reasons and maybe a forte oil flush.If this doesn't work it's new rings I'm afraid.