Hi all, looking for some help with a vacuum advance issue. I bought a 1969 2a 88" over the winter, got it running, registered, and back on the road this summer. Last week while out for a drive, it started running erratically. It was idling fine but whenever I opened the throttle, it would start to lurch with the engine revving high-low, high-low in a regular pattern despite steady application of the throttle. I was able to get it home lurching and backfiring the whole way. I checked leads, plugs, timing...everything looked fine. The next morning it was behaving the same way. It fired right up, idled smooth, but continued with the high-lo revving under throttle. I tested the distributor vacuum advance unit by disconnecting it at the intake manifold and applying suction to the open end. The distributor base plate moved as expected but the diaphragm seemed to leak as the baseplate quickly returned to its original position. With the vacuum tube plugged, the truck was back to running like a champ with just a hint of a stutter here and there. I ordered and installed a new vacuum advance unit and with it connected, I'm right back to where I was before...smooth idle with high-low revving under steady throttle application. With the vacuum advance disconnected I was able to take a 4+ hour ride through central and western MA this weekend that included both high-speed highway stretches and low-speed bumpy, rutted dirt roads. It ran great in all situations. Have you seen this before/got any advice for what to check next? Thanks, Glen.