Hi
I have the very same problem with engine power, driving the disco on its own it's not to bad, as like today coming down the M1 towing my caravan ( 24 foot twin axle ) as i hit a hill i have to drop down the gears ,may be down to 3RD on some, and at the same time the engine temp rises quite high, and when on the flat its OK but no real power. someone has said if may be the timing.
filters oil and air new lift pump changed sedementor bypass, and checked all the above.
thanks Martin