Firstly I would try and recalibrate the laser height, open the console, take the top off the cd tray and you should see either 1 or 2 while cogs (depending on which model you have) I just had 1 on my V7, but I suspect you may have 2 on your slimline. Ok this procedure is trial and error, take the laser too far either way and nothing will read. Just play around with it untill you get a happy medium (note/mark the cog/s before adjusting, so you have a reference point to where you started) also it maybe a good idea to clean the laser eye with some cd cleaner fluid or similar with a cotton bud and make sure the eye is dry before use.
However if recalibrating the laser position does not cure your problem, I would be inclinded to guess that either your laser is on its way out or that the dvd reader part of the modchip is at the end of its life, hence reading cds but not dvds.