caps have a vent, with a little cover over it. there shouldnt be anything making that much presure inside the cap, unless you are getting crankcase pressure leaking up the shaft, into the cap. i suspect you'd see a lot of oil coming up and into the housing, if this was true. snap the cap on and then try to lift or wiggle it off. it shouldnt really move. of it does, something isnt right. i've had junk caps, that just wouldnt mount properly, and could be moved off the dizzy, when clipped down. replacing with a quality cap fixed it for me.

you didnt mention engine size, but if you have a small block, make sure you dont have a broken motor mount. the engine twisting when torqued from a stop, could drive the dizzy into the firewall, and pop the cap. (depending on clearance in your particular car)