Could not authenticate with OAuth



i use the TwitterOAuth class in php for tweet news, but it does not work well.
sometimes goes everything good, sometimes i got this message: “Could not authenticate with OAuth”

I use the following settings:
Access level Read and write
About the application permission model
Consumer key *********
Consumer secret *********
Request token URL
Authorize URL
Access token URL
Callback URL
Your access token

Use the access token string as your “oauth_token” and the access token secret as your “oauth_token_secret” to sign requests with your own Twitter account. Do not share your oauth_token_secret with anyone.
Access token ******
Access token secret ******
Access level Read and write

Request settings
Request type: * GET
Request URI: *
Request query:

in the php source:

$tweetMessage = “blablabla”;

		echo "<br>Fehler bei der Verarbeitung! ProjNr: ".$row[ProjNr]." Fehler:".$ret->error."<br>";
	} else{
		mysql_query("INSERT INTO twitter VALUES (now(), '".$row[ProjNr]."' )");


and i like use this program from browser and with cron for timed running.

please send my any tips, what i did wrong!