Engine is under load will start to shudder running extremely rough

Hi guys I have taken it to 2 guys one a mechanic and one an auto electrician but neither have been able to diagnose the problem successfully.

The symptoms are as follows:

1 - When the engine is under load i.E on the motorway or driving up a hill the engine will start to shudder running extremely rough, even when the car slows to idle the engine will be shuddering and surging, sometimes the engine will cut off entirely, if you turn the car off at this point and leave it 10 minutes most of the time it will start as normal

2 - when driving at around 2000rpms not putting the engine under load the car will run relatively fine until it heats up to operating temperature then it will start to idle extremely low and surge and there is a loss of horse power, once again if you turn the engine off and give it 10 mins it will start back up fine.

What i've done so far:

1 - new spark plugs and leads
2 - new coil pack
3 - new air filter
4 - new fuel filter 5 - new fuel pumpNone have done the trick,

any help would be appreciated,

Thanks

Daniel