Red and green coolant can coagulate causing blockages in the cooling system. If changing from one to the other the cooling system must be thoroughly flushed with water.

Modern thermostats open around 88 centigrade and close about 84. Most things now days run at about 90 normally. The paso was one of the first water cooled bikes for Ducati and was based on an aircooled design. Therefore it is likely not to work so good as a modern bike and be less stable.

When my thermostat broke on my bike I swapped it for a new Aprillia 1000 thermostat which was very stable, 90 centigrade all the time. Then put in a NOS thermostat that I found on ebay and now the temperature is not so stable, presumably older technology.

Also, I would say that the gauges on 907/Paso are not the most accurate.

From what you describe that the fan cuts in and out, I would say you do not have a problem with the bike and it is just that your temp gauge is inaccurate which is causing you to fear the worst.

You could change the coolant if it is more than 2 years old (blue/green) or 5 years old (red). Do not mix different types of coolant or it will react and gel.
I use Havoline long life which I buy at a DAF truck dealership because you can get 1l, 5l or 20l containers real cheap compared to car accessory shops. It is designed for high stress engines so it helps maintain excellent constant temperature and has great anticorrosion properties. Do make sure that the concentration is correct. More water = better cooling but less frost protection. Do not go below trhe minimum concentration as there will not be enough corrosion protection, do not go above maximum or it will not cool effectively.

Enjoy the ride

one should check the specifications of the coolants to see if they can be mixed or not.
Unfortunately the colour is not a clear and reliable indicator. There`re red, green, blue, pink, yellow etc coolants and some car manufacturers have them dyed in a different colour. So it´s possible a coolant is sold by its manufacturer in blue but a car manufacturer sells it under his own brand and dyed red. (Alfa Romeo seems to do this)
Some coolants are silicate free others not and these should not be mixed or as Derek & riascns said it can gel or create flakes.

G.

edit:
http://www.machinerylubrication.com/Rea ... ndamentals
http://www.popularmechanics.com/cars/ho ... ts/1272436