I guess IainW nailed it!
I measure 12V on pin 3, but when I start this drops to 6.3V, which isn't enough to close the solenoid on the starter motor
Leading 12V directly from the battery to this pin 3 will feed the solenoid command with strong 12V power and solve the problem.
I'm gonna try it, because i have the same problem, I guess the circuit going to pin 3 is too long with too much contacts.
Thx Koen