...and if it knocking check both the knock sensor has not shifted and have a squiz at the crank angle sensor too.
As the guys have said there is a swag of fault finding intel on LS 1 forums...you do not have to be a member but simply agree to their T & C to view relevant posts.
One would hope it not a simple as old fuel with degraded octane or water contamination...a fuel sample and test will answer this possibility though.
