The uncheck border tip usually applies to the 3D problem with Win7, but it was worth a try.
Maybe its time now that you contact PTC support with that problem. If there is a solution please post it here.
http://www.ptc.com/support/mathcad_supportCenter.htm
http://blogs.ptc.com/2011/09/22/how-to-use-the-brand-new-online-mathcad-support-center/