to find the media id code download nero cd-dvd speed and insert a blank disc. then click on the "disc info" tab and tell us what it said under "media id"
memorex is very unpredictable media. your media code probably will denote that you're using poor quality media.
you haven't specified what burner or firmware you have. sometimes a firmware update will help improve burns on poor quality media if your firmware isn't already up to date.
sometimes slowing down the burn speed will also help. I would suggest 12x or 8x on the crappy media.
ultimately, the best solution is to obtain quality media. taiyo yuden and verbatim are the most recommended on this forum. TYs can be found online at www.rima.com among other outlets, and verbatim can be found in most stores (and will be on sale at best buy starting tomorrow)
i would look for a firmware update and slow down your burn speed. if these don't work I would switch to better media. sometimes you can get lucky with memorex, but they're problem is that they outsource to whomever is cheapest that hour. they have more different manufacturers than i have underwear (and i'm a 21 year old girl with a victoria's secret credit card )
get some reliable media and I promise your problems will disappear!