When you see enq: US - contention on Top 5 wait events on AWR or Statspack report or in ADDM findings you can take the following actions :
1) Check undo advisor if in 10g anf up.
2) Edit your undo retension parameter
3) Give space to Undo Tablespace
4) Move Undo tablespace to an other disk not so hot disk.
Mostly this wait event concerns autoxents on undo tablespaces.