It could be a number of things. TD5's have a lot more electrickery / sensors on them than the engines that I'm used to working on, but there are a few pretty standard things you might want to check:
1) Remove all air intake hoses and inspect them internally for any signs of delamination or collapse. If a hose is just starting to collapse, it can be sucked flat before the turbo boosts (i.e. at vacuum), preventing the engine from getting the air it needs.
2) Check for air in your diesel. The tiniest leak along your fuel lines can let air in, but not necessarily leak diesel out. In the case of your TD5, the fuel line is under pressure immediately from the tank, so it could be the in tank pump that is at fault (fractured?). I have had a problem with my 200tdi up until earlier this week, where air was being drawn drawn into the fuel line along a line that is under pressure.
3) You could have a temperamental fuel pressure regulator
4) You could have water in your diesel
5) You could have a clogged fuel sedimenter (if TD5's have them)
6) You could have an injector that is on its way out. It wouldn't necessarily show its self in any engine diagnostics testing either.
I take it that the specialists will have taken a look at your fuel and air filters?
-Tom