Since a new temp. sender switch was fitted onto my 95 wrx it has developed a missfire and is cutting out when driven with the engine cold. It starts ok and is fine once its hot. Any ideas please.

 

I think there is two temp senders.. one for temp gauge and one for the ecu.. if the ecu one is faulty then you usually get a check engine light and the fans on the radiator will be on from cold and it will drink lots of fuel (they all do that LOL)..
if the other one, possibly which you had changed is faulty then the gauge doesn't work..

Doesn't sound like either, as it is a 95 it has coil packs, have they been changed?, they seem to break down and need replacing after about 5years or 100,000miles roughly.
Either that or the plug gaps are to wide or the plugs need changing..

Mine miss fired when cold and ran on three cylinders and had a missfire when hot too, was the coil packs... it only had a miss when cold to start with..

JGM