freelander k series lacking power

hello
i've got a k series freelander that over heated due to HGF, the headgasket was replaced along with the head being skimmed and pressure tested.
the car starts up and drives perfect when cold but when it's up to temperature it starts misfiring/back firing and loses all power?
a lot of parts have been replaced to try and sort this out but to no success.
the replaced parts are:
fuel pump
throttle potentiometer
coil
plugs
coolant temp sensor for ecu
engine ecu
the car is a 1999 with just over 100k on the clock
any help with this will be gratefully recieved

As the heads just been off I'd guess cam timing is out a notch or two. It's close enough that it's ok for the warm up while the ignition is retarded but no good once it's reached normal.

You've changed the coil, and the plugs. Try the HT leads, if these are still the original it's highly likely they're coming to the end of their life. Heat can damage them especially the original ones due to the type of construction they are.

I've recently invested in some 8.5mm Magnecor HT's, the difference is actually quite noticable after the originals not being changed since the car was made.