I have a Honeywell RTH230B and sometimes it will cycle on and off for an hour. What's wrong and how do I fix it?

Programmable heating and cooling cycle lengths: 10, 12, 15, 20
Or 30 minutes.

Read all about it here. Left side under introduction.

http://customer.honeywell.com/techlit/pdf/69-0000s/69-1800.pdf