I have the same problem. I took the visor apart and found that a tny part of the (flat) tension spring had broken. I do not know if this means a complete visor replacement or, if the spring can be obtained separately.
You may have more look checking this than me – I cannot even get the recommended spark plugs here in Mexico! Please let me know if you have any luck.
Bringing this back from the grave.....this is the most annoying thing in my P10....I found that the plastic piece into which the visor clips into is really wide now, so I have to shove paper in there to put more friction, but it eventually falls again.
I took mine apart. There's a tough little clip that SQUEEEEEZES the shaft which has a flat in it. That shaft is plastic over metal. That plastic wears out and grooves and the flat goes away, so there's no longer a detent.
I tried filling and shaping that groove in with JB weld, but it's too soft and it gouges up pretty quickly. I'm wondering if I make a nice strong metal replacement if I could sell a bunch? :^)
Sure sounds like it... My passenger visor is nice and snappy, but the driver one is now out...
Nothing like the visor dropping in your face at 100+ mph diving into a braking zone at the track to get your blood pumping!