Hi Tony.
I reckon you're measuring something incorrectly.
In case you don't have one, I think the wiring diagram on page 364 of this PDF should match your 2000 model.
https://guzzitek.org/gb/ma_us_uk/1100/V11_1999-2003_Atelier(Compil-GB-D-NL).pdf
Anyway, there are three contacts on the TPS. Two of them are across the complete resistance of the TPS, and you should see very close to 5 V. between them.
The third contact is the slider that delivers a "split the difference" voltage depending on where it is along the resistance. When it is at the closed end, you should see said 157 mv between it and one of the other contacts. Between it and the contact at the "full open" end of the resistance, you should see nearly the full 5 V when the slider is in the "fully closed" position (spilt the difference, 5V - 157 mv...).
If you are only seeing 2 to 4 mV, and that also with a new TPS, I strongly suspect that you are not getting a good measurement, maybe not making contact with the probes or something.
How are you measuring (making the contacts)? Have you got a breakout to do the measurement, or are you sticking pins down the side of the plug, or how?