Re: 156 JTD 16V Sluggish!
What was the filth in the intake system? Oil? If so, I reckon your turbo's bearings are knackered so it only spins up when the engine eventually gets up to 3k rpm. Also, where was the filth, before or after the turbo?
Any other symptoms; misfire, smoke (what colour, black/blue/white?), high or low fuel consumption, any oil consumption?
It could be anything really - MAF, fuel pump, injectors, ECU...
I fear it will be expensive in parts, labour or both.
Best of luck though,