Bad gas or jumped timing?....take a guess

[swb85]

Alright everyone, put in your vote. I'm arguing w/myself as I post this, so bear with me.
Yesterday I found a quick shop selling gas 15 cents cheaper than everyone else in town and I was empty so I filled up and drove home. Now today the truck is pinging like crazy under any kind of load. Bad gas right??....Well it's suddenly bucking quite a bit under light throttle at lower rpm in every gear (4 spd). Under normal crusing on the highway it runs normal and I can give ~1/2 throttle without it pinging, but any more and it pings bad. So when I got home for lunch today I shut it off and tried to start it right back up. It didn't turn over smoothly, jumped around a bit but eventually started....made me think waaaaay too much timing....bad gas wouldn't do that. Checked the dizzy and it hasn't jumped, hold down is still secure. Didn't have time to pull the cap. After lunch it started right up smoothly, but idle was jumping around quite a bit. Same symptoms on the way back to work.

The motor is rebuilt 350 (PO did it) and only has 12k miles on it. After hearing the pinging, I was REALLY hoping it was just bad gas....but now I have a sinking feeling the timing jumped internally with the rough turning over when hot and the bucking. The PO was good about replacing everything on the rebuild, so I'd have a hard time believing he didn't replace the timing set (I've never pulled the cover, never had to set the timing). The thing ran flawlessly yesterday....

Thoughts?...Opinions?....Ideas?.....Suggestions?
Thanks guys.