ADY engine problems

[RedOne]

Can anyone offer any advice ? I have a 96 Sharan with an ADY engine, done about 150 000 miles. It has a problem that I just can't get to the bottom of. When driving at a constant speed, around 4000rpm, there is this very slight hesitation every now and again, which seems more pronounced when going up a very slight incline. There also seems to be days that the car runs very well, pulls nicely, but then other times when it's like the hand brake is on slightly, sluggish and sometimes hesitates while accelerating.
I've been to the local mech and so far he has replaced the Hall sender, plugs and lately, the Lambda probe. I have also replaced the plug leads with a Bougi cord set, the Knock Sensor and the Engine Speed Sensor, cleaned the throttle body out till it shines, cleaned out the AMM, all to no avail. I recently got a Vag-com program and read the fault codes and keep getting "00525 - O2 sensor implausible signal - intermittant." Disconnected the Lambda at the plug underneath and checked the Lambda heat voltage with the engine running and get volts that vary between roughly 1 and 10V, this varies very quickly, up and down every second, don't know if this is normal ?. Have checked the continuity between this plug and the ECU and all appears well.
I also have fairly hard brakes but have replaced all the vacuum lines and checked around the manifold for any leaks, all seems fine there. Am at a loss what to do/look for next.
Heeelp !!

[Crasher]

You need to replace the Lambda sensor and also the AMM now that you have killed it by cleaning it. Why is this in the Golf section, the ADY engine?

[RedOne]

My apologies but there appears to be a lot of information in this section regarding the ADY engine as opposed to the Sharan section, that's why I posted my query here.
I don't think that the AMM is damaged, I opened it up, removed the cover on the probe and sprayed the probe with contact cleaner, also cleaned the gauze screen which seemed to have a slight build up on it.

[Crasher]

Try replacing the lambda sensor first but they normally fail or give bad readings due to a failed AMM and they are very difficult to diagnose. The only sure way I know of is fitting a new genuine VW exchange AMM. In my experience, cleaning the AMM stops them working but the hot wire type are less prone to damage than the hot film type.