What was the temp gauge like??, when the big viscous fan cuts in because it starts getting hot, it sucks the life out the engine. had the same problem. when you go along the flat the air starts to pass through the engine again and cools it down, viscous cuts out then you regain power!
the pipes in the rad go up and down over time the silt up and the rad becomes inefficent. worth replacing only £150ish sorts the problem.