I've almost the same problem with two clients.
In the first, the old server was changed for a new and stronger server, with high performance. When they to that, and also installed oracle, the oracle performance was worst than in the old server. I searched for the parms of old server, and saw that in the new, less memory was allocated to oracle. Then, I changed the parms to became equal. For a few days oracle works fine. But now, I've received a mail telling that the performance slow down again.
In the second client, same application, but a little different study case. They've changed 9i for 10g. It is in automatic memory management. Also in this case, now the performance is worst than in 9i. Analyzing OEM, I saw that oracle is having so much I/O operations. Some updates and inserts was taking a long time to be completed, using so much I/O resources. I know that this can be an development problem. But I have a remote connection scheduled with the client in a few hours, when I'll try to change automatic to manual management, increase block_buffers, shared, and max_size, stop and start oracle server to see if the problem will be resolved. How can an application work worst in 10g than in 9i?
But, in case that this not work... I'll be without any others ideas to solve this problem... someone can help me with some tips if the actions described above doesn't work??
Thanks in advance.