Losing admin permission after a while

Hello!
it's a few days that i am experiencing a problem with my macbook pro.
i have mac osx snow leopard fully updated (i use to keep it up to date).
the problem is that after a while (maybe one hour or two) i am using the OS (doing several tasks) i lose the admin permission! or something like that...
let me try to explain better...
• i can't empty the trash cause i get an error about permissions
• if i try to open an application which requires to log in as admin (usually appears a normal popup with your name and a blank field where to type your admin password) it starts to bounce in the dock and then it quits by itself!
• if i go into the system preferences and i try to change some setting that requires to log in as admin, when i click on the lock (as usual, you have to unlock it before to do anything) i get the "Authenticating..." label next to it, and then it appears again the text "Click the lock to make changes."

What happens?! If I restart the system, then everything works fine, but in an hour or two it happens again!
Don't know what to do, really...

Please, if somebody has an idea help me!!

Thanks a lot

Repair permissions via Disk Utility (in Utilities)

Hi northrnchimp,
thank you for your answer.

Unfortunately I tried with disk utility, but I am not able to do anything.

Let me show you.

Boot from your install disc (hold down the C key as you boot), then select disk utility from the menu, click the first aid tab, select your OS X volume, then repair the disc from there.