(Excuse my English, it�s not my native language. I�m Greek, living in Crete.)

History:
XT 660 R, 2004 model, problematic TPS was changed by Yamaha for free, good condition, 30,000 kms (19,000 miles), prior owner is a gentle and careful rider. I bought it 10 days ago. I visited Yamaha authorized service, where they detected a very slow idle speed (around 1,000RPM) which they raised to normal.

The issue:
If we divide throttle opening in a scale from 0% to 100%, the bike runs fine from 20% and above. Real power, excellent acceleration.
The problem appears under this 20%, when i.e. I want to keep a steady speed of 70 or 80 kms/h, (47mph) which is my usual way of driving. Engine goes up and down, as if the injector doesn�t know how much to spray. The feeling is that engine does on-off continually.
Things get worse when I close the throttle a little bit more (not entirely), let�s say to the 5% of this theoretical scale. The ups and downs become severe. The bike trembles. Turning the throttle off (0%), this �sawing� stops, but the bike decelerates more than I wished it to.

Some solutions I�ve read around the web:
Use better fuel (100 octane�s)
Enrich the gas-air mixture
Change air filter (mine is brand-new)
Decrease the input of air temperature sensor on purpose, so ECU senses �cold�

I don�t agree with any of the above and I can�t accept that this bike was constructed to behave like that. Any ideas -or links to similar previous posts- would be very helpful to me.