Why will my xbox360 wont read some games or dvds?

i have an xbox thats out of warranty, which i bought off crap bay, it is my childs who plays it all the time, but lately it has stopped reading some games and dvds, at one point it would not read them at all and the 'open tray' error kept showing. i looked up some guides on cleaning the lense etc so went about taking it apart to find i hadnt the right torx bit, so i put it back together and somehow it worked ok but then stopped again, i got the right bits and got into the lens and cleaned it carefully with cotton buds, i also had to hoover out a tonne of dust that was inside the casing. so ok i put it together again and still it wouldnt read the discs then later it would and then it wouldnt, last night i watched a dvd on it, today it wouldnt load anything and now atm my son is playing halo 3 after about 15 attempts of reloading it and it displaying scratched disc mid game. i have just tried COD 44 which is just opened out of the box but i got the open tray error again, i reloaded halo and as he is now playing it that tells me the laser/lens is still working obviously, but why is it so hit and miss? also ive read about the heat receptors that can play up if they get too hot but i have a cooling system. i almost threw the console in the bin the other nite with the intentions of buying another as i thought it was totally wrecked but as its not , what is the problem or what can i do to fix this constant hit and miss disc reading dabacle?