You need to be able to jump out and look at the carburetor and see if it is really open or if the timing is advanced, from there you can decide what the fix might be. So next time it sticks, pull over without blipping the throttle and take a look.
If the timing is slightly advanced, you will see this problem. Light springs in the distributor may not be pulling the mechanical advance back 100% which will cause the higher rpm. The blip of the throttle is just enough of a jolt to get them moving again. It takes trial and error to get the right springs and weights to come in and advance to the right point. I rebuilt dad's old 1950's Allen distributor machine to run 12volt distributors.so setting them is a breeze.
As a quick test, run around with the vacuum unit unplugged at the distributor, plug the hose. It may not tell you anything other then it's not the problem.
Try and duplicate the problem by slowly raising the rpm in the driveway and letting off the throttle. If you can get it to "stick", throw a timing light on and check it.
