[Recon101]

if it has been a while since your last rebuild your timing chain may be loosening up. or you could have a couple of teeth broken from the wheel.

Recon101

 

[SAC69]

If the distributor hold-down is not tight enough, the dist could work its way loose and affect the timing.

 

[Guest]

If your distributer is wearing out it will become sloppy and the timing will change. I had this happen with a 6 cyl 72 Olds Omega. The bushing in the distributer was worn and developed a lot of play.

 

[Teebone]

Your dist. could be worn and has too much play
Your timing chain could be stretched
Your dist. plate inside could be moving
Your hold down bolt could be loose..

 

[rickoshay]

As the points wear the dwell angle will change. This can also affect the timing.

 

[brianl03]

Loose bolts...or [color:red]Gremlins?</font color=red>
-Brian