Same problem happened to me.
I used logminer to see what was in the logs and narrowed it down to an object which kept cropping up in the logs.
I sent this info to Oracle and I was told that the object had a permanent lock on it or something like that. gave this information to our application developers who were able to resolve the issue (they were the cause of the issue in the first place).
The best thing to do is mine your log or dump it and investigate what is in them. Take it from there.
Info on how to do this can be found on Oracle Metalink.
Once you have eliminated all of the impossible,
whatever remains however improbable,
must be true.