if the leads are starting to break down(common on mondeos) a weak spark will be the result, before buying new plugs leads ect try gapping the plugs at 1mm instead of the recomended 1.3mm to see if there is an improvement.
I had same problem which turned out to be the coil pack.
My friendly local moto shop let me borrow ht leads and as this made no difference i put in new plugs still making no difference.
Then a banger racing friend of mine suggested the coil pack, 'about £50 off e/bay' and after fitting, this cured the problem.
Hope this helps