Weird...cam problem maybe?

Could it be something as simple as a bit of water in the gas that passed? Putting Hydraulic lifters in a non-hydraulic block is never a good idea. They will be starved for oil and won't pump up. The use of a hydraulic styled lobe with a mechanical tappet may (key word) have drawbacks such as not spinning the tappet to decrease wear or effecting the shape of horsepower and torque curves. But the engine will still run smooth. Call it a hiccup, and forget it until/if it returns. Given your checks, I don't see any cam problems here.