What causing timing to change?

[PHOENIX]

Info on my setup.
Stock 350 engine (I guess the miles at being around 60,000)
Proform (In know), distributor
with msd module and coil (thanks to a member here )
AC Delco plugs (about 6-months old)
Taylor plug wires, in great shape still

Anyway I recently had some problems with strange noise from my truck. I checked the timing and it was way off. What causing timing to change like this? I had it set at 10 and it was probably around 24 or so. Truck seemed to run fine at the 24, but when I returned it down to 8-10 it ran much smoother and a little stronger. What the heck causes the timing to change like that?
Also when I was changing the timing back down it would slowly go down 10...............9...................8. In about 15 seconds or so I could see it dropping down. I tried it a few more times and got it to stay around 8. Everything seems fine now, but I need to check on it later today or tomorrow. Once again, what the heck causes timing to change on its own? YES my distr. hold down tab was tight, I could not move it by hand until I loosened the bolt.

Thanks for your help!