Engine Splutter : Linked to new exhaust?

[SAnt]

Garage replaced exhaust from just after the cat to the back and since then I've noticed a small splutter when changing down gears from 3 to 2 but thought it was just something that might need a few days to stabalise. It seems to be fine when going up gears, only hear it when going down.

So now 2 weeks on and it's still there, perhaps slightly worse and then today I stopped and left it to idle the revs went up from 0.75 to about 1.10 and you could here it splutter, then it seemed to adjust itself back to 0.75. It did this for about 5 mins, before I set off again.

Could it be the new exhaust part (incompatible part?), or just coincidence? Any help greatly appreciated.

Thanks in advance