Drive Belt? 300e 2.6
 

I recently purchased a 91 300e 2.6 with 90,000 mi. To date, I love the car. Overall its in great shape and was fairly well maintained in the past. There are a few issues, but what do you expect from a fifteen year old vehicle.
Here is the current puzzler. Symptoms; SRS light started to flash on for a couple of seconds while driving. This seemed to always happen the same place in my commute, just as I passed the airport on the freeway. It never came on at any other time. I did a search on this and other forums and found two likely causes: voltage regulator or contact ring in steering column. I took the easy route and replaced the voltage regulator. This did not help and just before the replacement and since then the light is coming on more frequently and for longer periods. (I no longer suspect interference from the microwave towers at the airport.) Turning the steering wheel doesn't appear to affect the light.
My question is, could it possibly be the serpentine belt. The tensioner is maxed and the indicator is pointing almost all the way to the right notch so replacement is due. I only question this because the charging system checks out fine at least in the driveway. The clicher for me is that the light seems to come on with changes in highway speed and doesn't come on during city driving, only on the freeway.
Your verification or suggestions are greatly appreciated.

The belt is around $30 and you said it needs replaced anyway.