judging by the fact your pick slips every 20 seconds i'd say you have no reason to push it any further because you either psycho picking it or really locking your muscules, and probebly both, and that's not a way to alternate pick.
i don't really see why you'd want to push your speed so much, and i suppose that if you insist i'd suggest fast but at a comforteble speed frame, i.e. don't go for yngwie fast, go for Autark fast, whatever you are capable of, the better you'll get the faster you'll get without even noticing, but my best advice is forget about pointless speed pushing exersices and try learning songs instead..
every time you learn a new difficult song your overall skill improves greatly, and it's much more fun.