Alas, I can provide a manual:
http://www.qvc.com/footers/fth/pdf/V32069_Manual.pdf
And I can let you know if you contact customer service for the product, you well may end up sending $10, a form, and a request for a repair if within warranty. The alternative may be $40 to Amazon:
Pursonic S450 Deluxe Plus Rechargeable Sonic Electric Toothbrush with...
A wild guess is that the charger is failing or there is a loose solder connection, neither of which is easily fixed via DIY (although there is a video related to soldering repair on a competing brush) and the cost of having a tech fix it outside of warranty is prohibitive considering what you can can pay for one.