69 340 timing chain questions - help!!

Well guys,
I put the front of the motor back together WITHOUT MAKING ANY CHANGES AT ALL, and the car is running like a brand new car. I took it around the block after getting the engine up to operating temp in the garage, and it misses no more. No stumble, no pop, nothing but smooth acceleration. WTF?! I didn't change anything at all. I looked at the timing chain, scrapped the gaskets off, and reassembled the car. Now, if it STAYS this way is another issue. There have been times in the past that I thought I had it licked, only for it to resurface on the next drive.
Any clues??

George