Stash,
The vacuum caps may blow off on a hard deceleration, especially the thinned walled ones. A sure sign of a vacuum leak is a lot of popping that appears suddenly. I had some thinned walled squirrel rubbers on and knew immediately when one blew offâ€â€thanks to this site--. Members have advocated tacking on the squirrel rubbers with a bit of RTV sealant or even using a zip tie.
In addition to checking the squirrel rubbers, check periodically, with a wrench, the header pipe to cylinder nuts. These will work loose.