Let me see if I understand this correctly: you set the timing at (for examply) 12 degrees initial advance (at idle), verifiy it with a timing light, then drive the car. When you return from a 10-15 mile drive, you re-check your timing with a timing light and now it's for (roughly) 28 degrees intial advance? And you're positive the distibuter hasn't moved? Unless your internal advance mechanism underr the breaker plate is sticking,I honestly don't think that's possible. My car would not even run if it had that much advance at idle. Have you checked the advance curve with a light? I feel this is much more important. Set the timing at idle, then while still holding the light on it, increase rpm's to (about) 3,000 and verify two things: first check to see that your total timing is about 32-34 degrees total, and second that the timing advacne process is smooth and it returns back to the initial 12 (or whatever) degrees you set it at. It's possible that both springs have come off I suppose, that's the only other thing that affects timing advance besides the distibuter housing.