still a jerky 325i tourer

[M7GHR]

i posted a thread a while back about the poor running of my car. it jerks on a steady throttle whichever rpm your at. u dont feel it when accelerating although it doesnt feel like ime getting the proper performance from it, others i have driven have been much better.
i got some advice on the last thread so will say what i have done, new plugs,leads,oil and filters,valves adjusted,afm track adjusted,intake manifold gaskets changed, rubber hoses checked for leaks or perishing, electrical connections checked. the car is very clean under the bonnet and all the connections looked fine, compression checked too
anybody any ideas?
also, what is the lead that goes to one of the ht leads for?