I know Win 8.1 is not supported but I having some trouble with the FMC on random occasion. When I go to LEGS and replace a Vector from my next waypoint, 1 out of 10 times, I won't be able to replace the vector (doesn't do anything) or I will get program pin error. If I reload the flight, everything goes back to normal.

 

Normal behavior or am I doing something wrong ? I experience that going from KLAX 24L which is a vector departure.

I don't think that has anything to do with your OS.  I often, almost every flight, replace VECTOR with a waypoint and never had an error.  I suspect an error on your part, if you can reproduce this then please provide step-by-step procedure of how to do so and we'll try to figure out what is going wrong.

Like I have mention, It only happens 1 out of 10. Every single time, I had the same FMC programming. 24L to vector 250degree to SLI. Which is the typical SEBBY8 departure out of LAX.