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Hi, my car encounters a "flat spot" and hesitancy in accelerating under load , i.e. from a standing start or overtaking up hills at speed . I have had it into my garage for this and was told one of the H.T. leads to the spark plugs was faulty and was replaced, this had the car going a bit better but not much so then I was told to change from 91 octane fuel to a higher octane as 91 was affecting the performance. This I have done(98 octane the last two fills ) but problem persists. What could it be please ? Thanks.

Hi there,
There is a common problem that I would look into first, the inlet manifold gasket has a tendency to fall apart and cause an air leak which could be causing your issues.
If this checks out then maybe pursue the sparkplugs/ Ht leads as more could be at fault.
A little bit hard to diagnose without being present and driving the vehicle.