The condition you describe is known as isolated diastolic hypertension. It's not one of the types of high blood pressure which normally worries doctors. There's a very technical report about it here:
http://archinte.ama-assn.org/cgi/content/abstract/160/21 /3301
The important bit from that report says this: "Isolated diastolic hypertension, as assessed by home BP measurements, carried a low risk of cardiovascular mortality, similar to that found in subjects with normotension" . This, if I interprete it correctly, means that you're no more likely to drop dead than someone who has perfectly normal readings. So stop worrying!
Chris