Mondeo 2 litre zetec petrol?

Hi, 2005 2l zetec petrol (estate) has a problem returning to tickover straight away, tickover is fine but when i rev the engine the revs drop to just under 1500 rpm, holds that position for one second and then drops to tickover. It's the same when driving, when i take my foot off the accelerator i have to use the brakes a lot more. Cables are not tight and I've already gone round the throttle cable linkages with wd40 and sprayed the throttle shaft at the plastic housing. Thanks for any advice.