Ford: Check Engine Light, ford crown victoria, o2 sensors

Question
Hi,
  I have a 1992 Ford Crown Victoria with 75,000 miles that is just glorious.  It drives so nice.  The engine is very quiet and does not hesitate one bit (probably the smoothest running car I've ever had, it even runs better than my 99 DeVille!).  There is one thing bothers me a lot though.  When I take it out on the highway, and get up between 60-85 mph, the check engine is always coming on and going off at an interval of between 5-15 minutes.  I don't understand what that is because this car runs perfectly fine.  During local driving, it doesn't come on, only on the highway.  Any suggestions?

Answer
Sean  it sounds like your o2 sensors are slow in responding this is a common problem with a lot of cars . but first let me know what kind of gas are you using? Shell is the # 1  that causeing these problems. I have had customers change to bp and the problem went away.