1998 Citroen Xsara engine management light on

Car starts 1st time everytime. Recently the engine management light came on whilst driving,and now misfires every so often. I was told using 95 instead of 98 unleaded petrol can cause this. Not!! I have used 98 unleaded since (4 tank fulls) but has made no difference. Seems to be getting more frequent. Also the heater switch only works on full power, is this a matter of just replacing the switch or somthing more drastic???