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.

Jez
September 2012