I can help with pics if it's an AFB style Edelbrock or Carter. I currently don't have any Holley's but can borrow one from my Brother to take pics if you really need.
It's most probably just some gunk stuck in a passage from ethanol evap or old fuel.
A secondary thought is vapor lock, I find it interesting that it only happens on load which points to vaccum leak.
How do the plugs read?