It can actually be the wiring getting old and be next to impossible to trace. Do what's been suggested, or the bulb integrity device itself could be out of tolerance. It is 15 years old. Both my 245 and 242 would do this and no matter what I did it wouldn't stop. Good brand name bulbs may (or may not) be within closer tolerances to each other, so I'd suggest using a pair of new brand names first, but if this doesn't do it stop looking. You'll just drive yourself nuts.