XP 32b, SP3
Firefox 3.5.7, flash 10
2GB memory (laptop)
Vista 64b SP1
Firefox 3.5.7, flash 10
6GB memory
I have not had the problems that you've encountered. The map in question opened fine for me (but of course I am not able to edit it). I have done more complex routes before (more turns, longer distances) without any problems.
I just added another route (manually) as a test on my WinXP machine.
http://ridewithgps.com/routes/25354 Distance and elevation appear accurate & it's over 42 miles and has quite a few turns.