I've had my 788 for a couple years and it has always driven me crazy with the blade slipping in the bottom clamp.  I'd tried wiping the blades with laquer thinner, sanding the blade ends, sandng the set screws and anything else I'd read on the posts.  I'd talked to the DeWalt guys at the wood shows and they say they'd never heard of the problem before.  On one of the posts somewhere, maybe on this site, but I can't remember where someone said to oil the threads on the thumb screw.  I tried it and it seems to have fixed my problem, the blade is staying tight.  I'm in a high school and they guys are finally able to use it with me always on them about tightening the blade. 

 

I have no idea why it worked, but my blade stays tight now.