I had the same problem, and checked it with ODB which said to "change the petrol heating sensor"
# Changed and the problem continued.
# I fuel it with High Efficiency petrol 2 times, and got better.
# Then I fuel it with 85% petrol and 15% benzine ( hoping to "clean" the injectors) and got better results.
Anyway it's an old car, but I think is better to check and if it need, change the injectors.