[LX5liter]

Wow. 12.90 @96 mph? If that It sounds like you're definitly losing power in the top end somewhere. I think you are either running out of air, fuel or spark. My guess would be fuel. What do the plugs look like after a blast? Do you hear any misfiring?. Your definitly making power by your 60ft's,but your losing power at the top end. What kind/size of air cleaner are you using?
I don't think the problem lies with the tranny or rear end. If it was an automatic then maybe, but not a standard. And with your 60ft times I think your clutch is holding just fine.