Olivier,
The vertical play shouldn't be more than approx. 0.1 mm!
You probably ought to disassemble and clean your distributor and replace the shims and washers, and probably the oilite/bronze bushings as well. Check this thread:
http://www.pelicanparts.com/ultimate/Forum3/HTML/009677.html
There are fiber washers and shims and ither steel washers in the stack, but all must have worn thru or come apart for you to have that much play! See if this generic exploded diagram helps any:
