engine is bucking when a/c is on

[Gordy_Schesel at allianzlife.com]

I thought it was fuel with an insufficient octane level at first.  Engine
bucking occurs only once in a while in the higher gears but only when the
a/c system is operating.  This has happened previously when wife put in
regular gas without thinking.  I have always used premium Amoco gasoline.
Is this a timing (timing belt is 40k miles old) issue?

tia
91 200 20v tq 108k miles
91 100 quattro 196k miles