Originally Posted by Govert

When you put the selector in S when you are driving 55 mph, does it shift back to third gear?

You can also try accelerating using kickdown of with the accelerator pedal fully pressed (only with a fully warm engine), upshifting should occur at 4000-4500 rpm. You should still be in third gear at 55 mph. Releasing the accelerator pedal at that point should cause the transmission to upshift to fourth gear.

Does the tachometer show around 750 rpm at idle?