I have a set on my ZX10R which have about 3500 km on them and are about half worn out. No issues with cracking. I just installed another set on a different bike but haven't ridden it yet. No issues there, either.
I'd suggest taking the bike (or wheels, if they are not on the bike or if the situation is so bad that you dare not ride it) to someplace knowledgeable, e.g. Pro 6 Cycle who are the local Dunlop race tire distributor, and get them to take a look at them.
I'd suggest taking the bike (or wheels, if they are not on the bike or if the situation is so bad that you dare not ride it) to someplace knowledgeable, e.g. Pro 6 Cycle who are the local Dunlop race tire distributor, and get them to take a look at them.