A symptom of loose chainring bolts that I have noticed is sluggish shifting from the small to the large ring. When the bolts are loose, the chain pushes the ring to the outside first before shifting onto it. If you tighten up the bolts then the shift becomes far quicker. I've also had chainring bolts occasionally loosen on me; I now use Loctite on them, check them occasionally, and pay close attention to them if I'm having trouble making the small to big shift.