If the system's reading discs once in a while, you probably don't have to replace the GDROM, or buy a whole new system.
As Norton said, try cleaning the laser with a Q-Tip, and rubbing alcohol first before adjusting the pot. Use only a little alcohol, and swap the lens gently. Use the dry side of the Q-Tip to dry. You might not even need to do the following steps, but here we go:
Unplug your system, and turn it on once to get rid of any extra juice in the power supply. Remove the Dreamcast's top cover so you can access the GDROM drive. Once you have access, carefully unplug the ribbon cable from the GDROM's lens. From here, you should be able to gently lift the drive, and you'll see the potentiometer you need to adjust. If you don't have a multimeter, try adjusting 1/8 of a turn clockwise. Bump it up to 1/4 turn if necessary. Reassemble, and test. Good luck.