When going up a good size hill or under strain (lead foot) on flat ground, my 2000 Jetta TDI seems to instantaneously drop power (feels like going into overdrive on an automatic). It then has no power to increase speed, and looses a lot of speed the rest of the way up. [It did not used to do this.] If after the hill, I turn the car off and on again, the power is restored. If I start the hill in cruise control, it won't lose power in this way, and does not lose much speed. If I've not been on a hill or "taxed" the motor, the car has a lot of zip! But after this, it's dead on power, no acceleration in any gear.

posible clock spring under the steerin wheel

There is a "clockspring" wiring spiral in the steering wheel to accommodate the rotating steering wheel with controls on it. If this breaks, you may lose some or all functions controlled by stuff on the s-wheel.

blowen gasket?
fuel filter full o ****
injector failure due to build up
need oil change due to sludge in pan

check air filter. good chance it is clogged also the shut off valve could have serious carbon build up both of these items are easy to look at. also unplug the Mass air flow meter if power returns or is better good chance it will be that. which will have been killed by a clogged air filter.

You need to take a look at your fuel pump located in the fuel tank and can be reached by removing the back seat.

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Your voltage regulator supplies the path for the control lead of the alternator and varies the amount of current available to provide the magnetic field of the armature with the required strength. Measuring from this lead to ground won't show much voltage.
Apparently the regulator is not integrated into the alternator and is probably failed. If you can chase the control lead backward, you will find the regulator and these are mostly mounted on the fender well or firewall.

could be worn out piston rings!

just install a new fuel filter and you should be fine.

hi if its just second to thrid then the clutch is slipping on thrid gear look at the boxes oil level is important but look at the colour is it red and tranperant as this is ok if its brown its a sign of the clutches slipping i would try a stall test on thrid gear first and if its ok second gear [manually] see is the engine stalls at the correct rpm if you thinkabout it imagine you are in thrid gear in a manuual box and suddenly drop into second it would be hard shifting it could also be a faulty thrid gear solenoid yates210456

find your local vw service center and see if they can plug their computer in to see if there are any codes on the cpu. there may be one for an intermitent for the turbo. i ran into the same problem last month and it was the turbo veins are sticky so occasionaly it lack power going up hills. it did not cause the check engine light to come on, but there was a code on the cpu when they checked it. if it happens again while you are driving, pull over, shut the car off for 10-30 secs then restart it and see if it makes a difference. when it happened to me i called my guy and thats what he had me do and it was like a new car after that.