MSD Wires

I recently bought an MSD ignition kit (wires, distributor cap, coil) and installed it. It seemed to work fine for a few weeks, but I just noticed that the wires stick out of the head now. They fit fine at first, and I can pop them back in tightly. But when I run the car for a few minutes, the 2 wires on the ends pop out close to an inch, and the 2 middle ones pop out about a quarter inch. The ride seems a little sluggish, and my mileage went down a tad recently. I since put the old wires back in and they stay in fine. Has anyone else had this problem with MSD or any other aftermarket wires? If so, do you know how to fix it? Thanks.

Many aftermarket plug wire do not fit correctly. As long as they make connection, it dont really matter. My MSD wire fit the same way
http://files.automotiveforums.com/uploads/516431civicengine2.JPG

The wires on your car stick out too? The thing about these ones is that they fit fine for the first couple weeks, then suddenly after driving the car for just a few minutes, they'll just pop out. The only things I've changed since the wires were new spark plugs, and those are in tightly.