Why is this 360 pinging

Well...too much squeeze is more than likely the culprit.
Like Adam said, do a cranking comp test and report back.
Egr, if it has one is another place to look for function.
The 1406 has always been too lean ootb for all my experiences using it, so if its just surging...its it need a rod change, if its punting under load and not due to timing 'which btw 31 isn't too much"...then it needs bigger jets.