Had a similar situation but only on a cold start. Once it ran e-thing was OK till the next cold start. Would actually hammer the pushrods straight on a 2x4 and reinstall them. It was only the intakes. Dropped the tank, checked it, replaced it, pulled the head, did a backyard clean, lap, v-job reinstall and haven't had a problem since. All this due to the old nasty gas sitting for I don't know how long. Been running the 216 over 10yrs , it just won't quit. It was unreal how that bad gas lacquered those valves, when cold you couldn't move the valves with a 2 foot pipe. Sounds like you are having a problem with only one p-rod so you might want to check the guide clearance. If its too close, along with a slightly bent v-stem, a little heat and ........ good luck !
