A couple of suggestions:
I'm sure that you're burning your own CD's using MP3 files. I won't even rant about what a crap format MP3 is, but it's enough to say that it sucks a$$ and leave it at that.
Now, here's some suggestions, making assumptions that you're using your own burned CD's:
1: Scratches. I'm appalled at the lack of care some DJ's show towards their gear and CD's. Could it be your disc is scratched up?
2: Cheap CD-R's. People love the cheap CDR's, and who can blame them. But the fact of the matter is that they don't hold up too well, or at least the generic brands don't. If a major name brand is being sold cheap, hey, that's a good thing and go for it. Personally speaking, I only use premium brand CDRs for my clients and myself these days. It does make a difference. The extra cost is repaid by not having to deal with playback issues.
3: Along with cheap CD's are the surfaces. While all the surfaces(silver, blue, gold, green) are supposed to behave the same, some laser heads either have preferences towards certain colors or flat out hate other colors. Please keep in mind that burning is a chemical process, not a "burning into a substrate" issue.
4: Labelling. How are you labelling? Are you using a proper pen? Are you using adhesive labels? It's entirely possible you're causing problems by accident. The plastic layer is just that, a polycarbonate disc. ON TOP(the side that should be on top when played) has a thin coating of the photo-chemical material, covered by a thin plastic film to keep it in place. You're literally sticking or writing practically on top of the burned data. My suggestion: Convert to printable CDR's and then print your labels. Why? Well, the adhesive labels have a tendency for the adhesive to come away(peel back or even bubble) or dissolve through the film and attach the chemical layer. Short of that, sticking with good quality name brand CDRs and using an approved pen will be a good alternative.
5: Power. Are you taking command of your power situation? Are you using something like a Furman power unit or you just go into a cheap power strip and then into the wall outlet? Going a step further, a powre conditioner or UPS? It could be spikes and dips in the power that could be causing glitches. There could also be other devices on the circuit pumping noise into the circuit, which in turn is affecting your units. I know for a fact Furman power units filter out any RF and EM in the power, so there's a cheap troubleshooting tool(or expensive if you go with their well worth it higher end units) that serves many purposes and is something I recommend regardless. Every rack of mine has a Furman power unit of some sort in it, and that is NEVER going to change. Well, there is 1 exception: my electronics drum rack, but that is rarely used and there's a Furman unit on stage that front ends that anyways.
6: Speaking out of turn, perhaps you have some bad firmware in a unit?
7: Since it's unclear where you are, are you sure you're doing the power thing right? Is you gear or at least the power supply set up properly for your local power? Some of the power converting transformers(such as a step-down that goes from 240/50 to 120/60) designed for travel don't work so hot for more sensitive gear, such as your CDI 500 MP3's. I'd like to rule this issue out, prefering the local power isssues of dips and spikes mentioned in issue 5. Getting a better rated power converting transformer cost a bit more but is a well justified cost for those who truly need it.
Overall, how do they work with commercially produced CD's? If those work fine, then it's your burning or care of the discs(or both). Maybe it's the bitrate on some of the MP3's, which granted, you may not have control over. I can make my own MP3's, and therefore I am fortunate to have total control over a project from the writing of the first note, through recording and production, mixdown, mastering and any format conversions. It's highly probable that the unit may not like certain MP3 codecs and/or bitrates. It shouldn't have such problems, but it is a possibility.
What you might want to find is an electronics repair shop and/or a pro audio store that does repairs. They may be able to help you on getting things fixed. Since you are international, you might want to work with ADJ's support via email if they do that sort of thing. Please, do be as detailed as possible. Your posting left out a lot of desired details. If you want to continue troubleshooting out in public via this forum, then by all means do so. Perhaps some other kind reader has also experienced this issue and even better, may have a solution for you.