Intermittant Power loss

Hey all, new to the forum, and got a stumper here. My old trusty wrangler experiences a power loss on acceloration, at about 25 mph, when a loud hissing noise comes from the engine. I've checked all the vacuum lines, and they all seem intact. Sometimes when i drive over a big bump (which in a jeep can be just a dime) or railroad tracks, the power suddenly returns, and she drives like new again. Any suggestions?