backfire problem

[tim21391]

i have only been driving my truck maybe twice a week.... yesterday (sunday) i decided to finally tighten my tranny line on my radiator... then i noticed i had a spark plug wire that had fallen off...i knew i was running rough before so i plugged it back in...it now has a smoother ride...but now when i get up on the interstate i hit passing gear... and it back fire takes off backfires again and so forth...i noticed i had a little rust on the plug like it fell off a long time ago....could the backfire mean that im not getting a good spark and the tip of the plug needs to be cleaned...if so would it work if i took sand paper to get the surface rust off??

thanks,
tim