You asked "Please descibe this 'stalling' of the rudder upwind"...
I've never experienced this on a catamran but the same thing does happen when windsurfing, there it's call spin-out and it happens for the same reason - the "skeg" (which is like a fixed rudder) stalls... loses it's ability to develop lift and loses it's diretional stability. What does it feel like? It feels like you've lost your skeg, ie, like the rudder fell off... time for the warp factor six face plant if you're unlucky!
