At what RPM does it begin to stick ?? The secondaries open at ~3400rpm. If the "sticking" occurs at around that rpm, I would suspect the IMRC (Intake manifold runner control)

Everything else ok ?? Plugs/wires changed recently ?? Air filter clean ??

Bless our servicemen & women overseas. L.Cpl Ian Malone, 1st Battalion Irish Guards, R.I.P.