kangoo poor running

Hello,i've had a running problem with my 2001 kangoo 1.4. I know the car is electronic ignition but the only way I can describe it is as if the condenser is weak? Its worse when first started leading to a lumpy/jerky drive until warm then fine upto about 60mph with the engine service light flashing occasionally at 65/70mph. I have replaced the old rotten catalytic converter,spark plugs,air filter,petrol filter. Im going to try half a tank of the better unleaded next week in the hope it could have been a poor batch at the tespoo last time. Lets be honest and open about this.....the car is a bit of a shed but has been doing sterling work carrying me to work and back with very little love shown back on my part,im hoping to get it through the winter and then replace at my own leisure in spring. Ive spent £90 on it so far and biscuit tin now only has a single rather lonely looking tenner in it. All ideas and sensible suggestions listened to including investing said tenner at dog and duck