If your timing won't stay where you set it or it jumps around when the engine is running, check your distributor bushings. I had and engine that was like that, and the symptoms were just like you'd get with a vacuum leak, and it almost drove me crazy until I put a timing light on the engine and the timing wouldn't stay in one place. If your vacuum gauge gives you consistent readings, the dist bushing may be shot, so you might pull the distributor and check for play in the shaft. If it wiggles even a tiny bit, the bushing is history and the timing won't hold steady. At that point, it's probably time for a new dizzy.
