limp mode madness

[cookiemonster]

please help as this car is driving me crazy!!!!!
i have a 2001 (y reg ) 1.9 tdi 100bhp avb engine code , bellow 2500 rpm it goes in to limp mode, if i nock it out of gear switch of the ignition and restart it , it clears the fault .if i keep it above 2500rpm it pulls like a bull but the local police are not very happy . had the turbo and maf sensor checked and both are working as they should. the car is high millage but has been very well looked after (255000 miles ) and this is the first problem i have ever had . any suggestions