Not quite sure as to what you are doing and what is happening.
Start of by listing your system, in detail. Include the PR for the Venice, the specific ram you are using (there are TM Speed Premiums and the non-Speed Premiums). What information do the ram chips have on them. What PSU do you have, operating system, cooling and anything else that you can think of that might be pertinent. Are you doing the 2.5 with the Venice and the PC2700 or is that with another cpu? I couldn't quite tell from reading your post.
Describe just what steps you are going through in order to try to overclock your processor. Include BIOS settings for cpu voltage, ram voltage, memory timings and divider used, if any.
This information will help us figure out your problem.