1996 Ski-doo Formula SS, reverse gear slipping out?

[Adventure]

I have a 1996 Ski-doo Formula SS with reverse. When I am in reverse it seem to work fine and then slip out of gear. The reverse lever has not moved but I must disengage and re-engage the reverse shifter to use reverse again without slipping immediately. I am wondering if adjusting the adjustment rod to add more pressure may fix this problem (although it does not seem slack right now) and if I do adjust it how do I know how much pressure to apply.

Thanks,
Dave