If a blood clot in the leg comes and goes, does it pose a real health risk ?

DVT: Is a serious problem. When you say it comes and goes i assume you mean the pain and swelling comes and goes. Yes deep venous thrombosis can cause death, it is very serious. See a doctor.

Certainly can: Recurrent blood clots in the legs is certainly not a good thing and each occurrence not only damages the vein but poses the possibility of a pulmonary embolus which is the life threatening problem. The fact that they recur suggests a coagulopathy which should be evaluated urgently by your doctor.