okay, I have a 52 plate ST170 which has a kinda surging feel to it when doing any steady speeds ie 40mph in 4th gear or just generally holding a constant speed, also feels sluggish low down in the revs like its hesitatant to go.

I've changed the plugs, leads, coil pack, fuel filter, alterantor (was faulty anyway) and have yet to cure it, ive also had it checked at my local garage on the diagnostics plug and that threw up no faults or error codes.

Not sure what to check for next, can any one shed any light on the matter or have experianced simular ?