vecra di 98 looses power on hills when in 4th or 5th?

my 98 vectra 2.0di 16v, i loose about 10-12mph on hills on motorways when in 4th or 5th gear seems to lack power after coming out of 3rd gear recently changed map sensor, cleaned and checked egr system removed inlet manifold and cleaned all the gunk in there and around butterfly valves, give some diesel majic and give it thrashhing, problem still persists, its had full service and 120,000miles