Shifts Abruptly Into 2nd
 

I have a 1999 Cirrus with an automatic transmission. Every once
in awhile when accelerating from first it abruptly shifts into second.

I had the fluid and filter replaced and checked at a transmission service
center but they couldn't find any problems. I figure if it was a mechanical
problem it would do it all the time. Is there a sensor that controls when
it shifts ?

Thanks