Dear Jack,very simple,the heat sensitive bi-metal spring in your thermostat has lost its' snap-back ability (old age). The whole unit costs about $8 at your local plumbing supply store and all you need to replace it is a #2 phillips screwdriver & possibly a pair of needle nose pliers. It should take you all of five minutes. If you want to make a radical change, you could install an RA-28 Seisco tankless waterheater and never have the problem again & never run out of hot water again. Call or E-mail me for info. Try www.seisco.net then please call me to order one @ 800-851 9891. I also sell Gas Aqua-Stars & magnetic water conditioning for scale control. Cheers,Brian.