This problem is probably the first thing people noticed about their new modded box, its not the chips problem, its the dvd-rom.
The DVD drive is very picky about what disks you use and occasionally will not load any game for a while (5 minutes). Mostly its to do with the laser in the drive, writable cd-r's and dvd-r's use a dye whereas original dvd's and cd's have the pits or gaps physically imprinted.
side on magnified view of a writable CD-R or DVD-R
side on magnified view of an original cd or dvd
DVD-R and cd-r/w drives have a laser which is hot enough to burn through the dye so that in a normal drive the reflective layer reflects the reading laser at a different angle or wavelength, which means the hardware can detect a 1 or a 0 (as in all binary data).
The same process is used to read an original disc, but as the original has 'physical' holes or gaps, there is a bigger reflective difference between the 1's and the 0's, which means it is much easier to read.
Quite a few standalone DVD players cannot read DVD-R discs because of this same problem. You need to have a better quality of laser and reading eye to be able to read writable discs.
M$ didnt need to spend more money than needed just to get the xbox to read writable discs as there is no need (or want) of it.
Basically, the short and the tall of it is this...
the drive in the xbox is crap.
There are a few things you can do,
you could install a pc-dvd rom in your box Here
you could try different disc types and stick with what works best.
you could get a dvd burner as dvd-r's work better than cd-r's
or you could adjust the pot on the drive Here
until then you could do what I do when a disc is being picky, eject the disc and close the tray with no disc in it, you can hear the drive go whhhiiirr whhhiiirr and then open the drive and put your disc in and switch the xbox off (the drive will shut), if you have evolutionX installed on your hard drive you would go to reboot on the menu instead of switching off, otherwise , just switch on and cross your fingers.
I have no real idea of why this works, it has come from trying every
permutation of disc in/out/reboot/power off etc. and choosing the one that works most. I guess it's because the empty drive makes the reading laser of the drive go back to the 'rest' postion (but I could be wrong, who cares? it works for me)
Hope this long winded ramble has helped a little.