maybe the timing gear is marginal and has "jumped" a tooth -- possible if its a fiber gear. Remove valve cover and bring up to #1 tdc and see where valves are at. The rocker arms should both be loose on #1. Or have someone crank it over while you watch for rocker arm movements as there could be worn lobes--see if all rockers rock about the same amount--wiped out lobe's rocker wont move much.
