Yeah, these kinds of problems are nasty to solve. The network guys will say its the server, the server guys will say its the db, and the db guys will say its the network :-)
Some good info here on how to setup SQL Net trace.
http://juliandyke.com/Diagnostics/Trace/NetTrace.php Note - the trace files can be verbose !
It may be the best bet to get as much tracing info as you can, then ask for assistance from Support to dig through it.
Hope this helps.