I have a problem with my automatic gearbox.

When driving along at 60 or 70, if you take your foot off the accellerator and put it back on, even very gently, the car changes down 2 gears.  It doesn't happen so much at lower speeds about town.  If I use the cruise control to accellerate it either stays in the same gear or only goes down 1 gear.

This problem only appeared when my exhaust started blowing, so i thought this might have been causing the problem, but I have just put a Powerflow stainless steel system on (it is quite quiet and gives good power) and the gearbox is a bit better, but not 100%. 

Is this a symptom that it is needing a gearbox software update?  Could it be my accellerator pedal?  It feels quite stiff.

Any ideas?