Sounds like a battery issue and the batteries in the handset need recharging by putting phone on base. Check the phone is correctly placed on the base so the batteries charge correctly.
...or the batteries may be nearing the end of their life and no longer hold their charge. Time to buy new ones. I had the self-same problem until I worked out what was wrong.
No, definitely sounds like a battery issue to me. Some people think that rechargeable batteries last a lifetime; they don't. Mine lasted 5 years before I had problems. Whatever you do, make sure that you buy rechargeable ones and not ordinary ones else you will knacker your phone.