Could be the battery is starting to go.
It's needed to keep the voltage up when the fans kick in--the alternator can't respond quickly enough to keep a voltage drop from happening when the fans kick in, so the battery acts as a big capacitor to keep the voltage from dropping.
You could check that the fans are turning freely--spin them
by hand (with the engine off of course), they shouldn't bind or feel gummed up.
Brian
