Twitter API forgets OAuth access tokens after some requests


I developed an application which automatically sends a bunch of different requests (~50) now and then. In every test run I got a very strange problem: the first ~30 of the requests are sent without any problem and then the API gives the error »Invalid or expired token.«
I obtained the tokens via (»Your Access Token«). Always after running the first ~30 requests, the tokens are not displayed anymore on and I got this error message. Is this a bug? I used the »Your Access Token« method for several different apps and it worked everywhere just fine. Do you have any recommendations?