Hey
In my opinion, YES - that 12 volt fan mod is affecting your DVD drive. Remove it. I never liked that mod. There are better sources of 12 volts on the Mobo - that won't effect performance
There are 4 voltage lines in the DVD power cable. Two are 12 volt, one is 5v and the another 3.3. Important there is not a drain on them. So patch your cable or buy a new one that is intact. Should be cheap.
Before you buy another Laser assembly. Remove the mod - then POT tweak the laser you currently have. Turn the DVD pot clockwise not even an 1/8 of a turn. If you have multimeter, take a resistance reading before adjusting. Then lessen the resistance 100 ohms at a time - the less than 1/8 turn
Other thing it could be. The dvd spin motor isn't turning fast enough. Could be the mod - Could be the motor is dying. You'll have to open your DVD player - plug it back into the 360 and watch it run. Remember the top half of the spindle - Very important to have on top of the disc before it starts to spin.
Let me know what you find - Or if you have trouble with anything