Your distributor could have slipped, but that may not be the case since I assume the shop that backed your timing down to 0 saw it was ok to begin with.
My car just started pinging like crazy. I think my distributor may have slipped. It was fine a week ago, now i hear the rockshower in 3rd gear. That darn distrubutor hold-down bolt on the 5.0 is wedged in there so that you can't really get a good grip on it and I think I may not have tighened it enough last time i did timing. Check yours, but I may also think like chadtn2 said, I may have gotten bad gas...like the station refill truck dumped 87 in the 93 tank by mistake.
------------------
Blueneonman -
'95 5.0L Mustang GT Convertible | K&N Custom Cold Air Intake | FMS 9mm Wires | Hollow Cats | Flowmaster 2 Chambers | BBK Subframes | 6-pt Rollcage | King Cobra Clutch | FMS Adjustable Clutch Cable | B&M Short Throw | Coolest Neon Lighting on a Mustang EVER!
Homepage:
http://www16.brinkster.com/blueneonman/