300 tdi power drop

I have a 300 tdi lump in my 1990 Disco and have just experienced a drop in power whilst driving down the motorway.

The turbo seems to have got louder and the power dropped off. I have a burton intercooler fitted.

Does anyone have any ideas as to what it could be. There doesn't appear to be any smoke and the boost guage that I have fitted seems to report normal turbo boost

Appreciate any help, thanks Scott