Is an elevated blood platelet count over many years a reason for concern if no other blood count problem?

Possibly. Essential thrombocytosis (high platelet count) is associated with increase risk of excessive clotting. The problem is worse if you become dehydrated. I had a patient who actually had a stroke attributed to essential thrombocytosis at age 41. Check in with your doctor soon.
Yes. You may have underlying blood disorder. It is best to talk to your doctor to begin investigation. Sometimes a hematologist may need to be involve if etiology can not be readily found.