help with 1999 transit acceleration problem?

my 1999 2.5 deisel (non turbo) tranny has for the last 12 months been developing a problem when hot, of not accelerating with throttle fully open, no smoke, no misfire,but just picks up ever so slowly then after say 500 metres in 4th it suddenly goes!, change to 5th and wait another 500mters before it suddenly accelerates. Always serviced, always starts first time. It feels like something is sticking then frees itself, have seen some possible links to a Plunger in the deisel pump top, but cannot find any more info, any help would be appreciated.Martin