FOCA

A remap or Bluefin would give you the biggest gain, Superchips claim 176Hp 387Nm -

  http://www.mybluefin...us20tdci136.pdf

If you ditch the baffles in your exhaust and switch them for a straight-through silencer and fit a decat,

That would make a bigger difference than an bigger IC, - and it would be a quick car  

I think the ST has 57mm/ (2-1/4") inlet/ outlet on the intercooler - i would expect yours to be the same

Sounds like it might be the DMF (dual mass flywheel) then - if possible get an expert to look at the car (you may be able to get a mechanic/ smaller independant garage to diagnose it for free, in order to give you a quote)- can be quite expensive to fix (for both parts and labor)

Might just be a belt slipping - does it do it more when the weather is wet?

Hello - nice one

I recommend you don't get that particular one because -

Its too big - its a mis-match for your diesel engine and your installation 

because it is so big, it has a large internal volume,(larger than your stock intercooler) imagine your turbo was a hosepipe, and your old intercooler a cup, and the big one a bucket - the hose takes longer to fill up the bucket with water than the cup - its the same with the intercooler - the turbo can only supply a specific peak maximum cubic feet per inches of air, the larger intercooler has a larger internal volume (for the turbo to fill up) so the lagger intercooler has a tendency to increase lag and reduce bottom end power, this may be ok for a petrol-turbo car but no use for a low-revving turbo-diesel

A larger intercooler has a larger mass of metal - this means it has a higher thermal inertia - which means it takes longer to heat up - this is good for petrol-turbo cars because they are only on high boost for (surprisingly) short periods of time, so the mass of metal/ high thermal inertia helps to stop the intercooler heating up too quickly (as long as it is well installed - more of that later)

On a diesel-turbo, the system runs much higher boost and the boost is high for longer durations - this means a high - thermal inertia intercooler (ie- a big one) takes longer too cool down,(especially if its badly installed, keep reading) this can be a disadvantage, and can loose power

Installation - on the Mondeo &  Focus, there is a large metal crash-bar (the bumper "proper") about 8"/ 200mm above the "deck" the stock intercooler fits in this space, it is low down (heat rises). Any intercooler higher than that is "masked" by the bumper (the cooling airstream does not flow directly onto the part behind the bumper), on top of that, the radiator/ AC is right behind this, and the un-vented diesel engine bay gets pretty hot - all adding up to heat- soak and loss of power

No part of the intercooler should come above the bottom of the bumper, the entire front surface area should  be in the cooling airstream

Pipes/ hose diameter - if you have a 50mm (2") pipe going into a 3" (75mm) you can get a sudden loss in velocity, this can cause turbulence, even if turbulence is not caused the lower velocity can reduce pick-up at low revs/ increase lag

Weight - the stock mondeo intercooler weighs 2kgs,  the upgraded one i fitted weighs 4.5kgs, this is the only heavier bit on the car - it fits under the front bumper, and i have a vented bonnet and an external waterspray

A large intercooler like that could weigh 10- 15kgs (a lot of wieght on the extreeme front of the car) - this can effect handling adversely

You would be better with an intercooler that matches the installation of the car (fits under the bumper/ fills the available space) and the diameter of the boost hoses, and the charecteristricts of your engine - if you gave me some info i would be happy to help/ recommend a more suitable intercooler