After a few minutes of playing a cd or the radio, the speakers make a thumping/crackle noise and the music stops. The stereo stays on. Just stops playing music. Any ideas?

Hello

OK, this is just a suggestion, without analysing the unit myself.

From my experience with these type of units, it is usually overheating of the IC's on the circuit board.
You can do another test just to double check. Turn the unit off, unplug it from the wall supply for about 10 minutes. Then plug the unit back in and turn it on. Now turn the volume very high, without the speakers starting to distorted, where you can still hear a clear sound.

Now, if this is an IC overheating, the sound should start distorting and cutting out. This is actually a very common occurrence with these types of players as well as receivers in general.

Try the above, let me know what happens.

Also, please do not hesitate to let me know if you need any further assistance.

Kind regards
Andrea