Audi A6 problems change Tiptronic

Hello everyone:
I'm writing to ask for your help. I have a 1999 Audi A6 2.5 TDI with a Tiptronic transmission, and when I start it cold, all the gears on the instrument cluster get locked (as if they were all engaged). Even when I switch to manual mode, the same thing happens, and the car lacks power (as if the clutch is only partially engaged). The thing is, after starting it several times and accelerating, the car resolves itself, and this only happens when the engine is cold. The garage I take it to doesn't show any error codes on their diagnostic machine, so I don't know what the problem could be. Any help would be greatly appreciated. Thanks, and greetings to all.