common problem on the 300tdi to blow the head gasket from the rear in no 4 cylinder
had done a couple and in drive wise yes would be slightly out of power but had never overheated
check the gear liver from top of the box
maybe needs some adjustment and check if there is alot of freeplay as the aluminum part get eaten from the sides
and again on top on the right there is the sping were holds the lever to stop maybe needs replace
sorry if cannot explane easier