Andi,
To begin troubleshooting this, I'd check the Oracle DB server for trc (trace) files, to see if an error there is causing the DB connections to time out or get cut.
If nothing is found at that tier, then check the network devices (NICs, switches, routers, firewalls, etc) between the OTM App and DB tiers, to see if there are any network errors.
Finally - if nothing else is found, you can configuring the DB connection pool refresh in OTM, to see if that will help:
http://www.otmfaq.com/forums/f30/tut...-refresh-1223/ (Tutorial: OTM - Setup DB Connection Pool Refresh)
--Chris