What can cause an erratic tracking problem?

My mouse pointer doesn’t track across a document or any file on the screen evenly, The mouse mat(s) have not changed but the problem is getting progressively worse. I changed the battery. I have tried cleaning the laser port cover on the mouse but it didn’t make any difference. I have two of these mice and they both have the same problem. They need repairing, can you tell me if this is possible, please? They are 9 years old and were quite expensive, about £50 each, I wonder if spare parts would still be available.