While In World Map with just a Departure and Arrival airport specified I try to add waypoints to complete the route I want to fly.

I click on a waypoint or a navaid (black triangle or black star) and place the cursor on ADD which puts a white border around the ADD box and I click and the waypoint/navaid is added to the route.

Other times (in the same sim session with the same waypoint/navaid I do the same procedure and see the white border but the waypoint is NOT added to the route.

The same thing happens with REMOVE - sometimes I can remove a waypoint and other times when I click on REMOVE (which has a white border around it) nothing happens and the route stays the same.

I can find no consistency in when or which waypoints/navaids are problematic.  

AND - I try thee or four times to add or remove with no good result and then try one more time and it works.

What is causing this and how do I fix it?

I do not remember having this problem prior to SU8.

I personally never use the default flight planner.  However, if the waypoint is part of a STAR, SID or Approach you should not be able to modify it.  You should be able to bypass it (via direct to) or modify specific parameters (alt) but not actually delete it.

Neither departure or arrival airport (both are just small GA places) have SID or STAR.  

None of the waypoints are part of any approach.

Some of the problematic waypoints, navaids are out in the middle of nowhere but I still cannot add them to route of delete them if I did somehow add them.