iphone 3gs sleep/wake button does not always work

This may be a silly question but have you restarted your phone? Press and old the sleep button till you can red swipe to shut down. If that doesn't work, press and hold the home and sleep buttons for 20 seconds till the phone does a forced reset. It may take a whole for the phone to boot back up so you may see the apple logo for some time after this. Be patient. Let me know how it works out for you.

A temporary fix for the Power button: is, in that corner squeeze (pinch) the front screen together with the back case while pushing the Power button. See if this helps. If it does, then some get fix if there is a separation in that corner, by carefully supergluing the back case to the metal bezel in that corner.