I would be checking all your hoses had similar prob with 3sgte changed out everything to eventually find a cracked vaccum line

Thanks everyone for your suggestions, I've finally found & solved the problem. Just in case someone stumbles over this in future, the issue was a faulty distributor, and the key symptom was a check engine light on beneath 2500 RPM, and off over 2500 RPM, but no logged code. All resistance tests of the distributor checked out as per the BGB, but swapping the part made the issue go away.