please help !

I've bought a 98 mondeo 2.0 GLX, - with a Zetec engine. It's completed 70 k miles with full ford SH. It runs hesitantly under load, especially so with the air con on. When the throttle is released it kangaroos a bit, and the flat spots seem to be consistant within the rev range (if it were on a dyno graph it would look like the tourqe curve dipping and then picking up in certain places)

I replaced the HT leads, fuel and air filters, TPS, cleaned the injectors, plugs and checked the oil weight.

I even bought and fitted a k+n 57 kit a I thought it might be air/fuel mixture, and I reset the ECu.

Any ideas pls.... the underlying point to me is the difference between ac on and off - it's far worse when running the AC.

Thanks

R