St170 hesitation from 1500/2250rpm at full throttle

[Warrior]

I have just bought a focus st170 03 plate 78k on it, with the idea of using the car for abit and possibly putting the engine in my Mk2,

got it not driving needed a cv joint, turns out somone has removed the engine managment light, having scanned it the only fault is a 02 preheat fault, could this be causing my problem

It did misfire like mad at idle and all rpm's but somone had washed the engine and plug holes where full of water. Dried it all out and leads and cleaned the plugs thought it was sorted.

i don't want to just start changing parts as I don't have a lot of money in the car and could live it as is,

it drives fine above 2250/2500rpm pulls well and hard all the way to the redline.

but if you give it full throttle from 1500 ish it hesitates badly, but will drive round at lite load fine.

ive searched but everything I've found on here and google the problem is higher up the rev range.

[Graham]

it wont be an 02 sensor heater fault.

could be knackered plugs