Did you find out the problem?
I also have H reg 300 SE of similar mileage, and strangely have similar
problem. I experience loss of power if I go up hill or above 60 mph
especially when trying to overtake. I can't get it past 60.
I had car in the merc specialist who checked fuel sysyem up to ECU and
could not find the problem. Someone mentioned it could be the torque
converter but I am bot so sure.
Hence I am very interested to know conclusion of your problem