Hi new member with a problem

I have a 59 reg. Freelander 2 2.2 diesel. This last month it has started losing power in high gear.
I have to get it to around 50 in third before going into a higher gear because it won't pull. By the time I get into 6th its losing speed and won't go above 50 and then I have to drop down through the gears again.
Tried two different blanking plates. With and without the hole. Its made no difference. Any body got ideas where I go from here. Its got 85 k on the clock. Thanks

Post in the Freelander forum?

For safer option worth getting diagnostic reading first before tampering.