It sounds to me that you have a restriction in your air intake or exhaust path, that is causing the motor to "run out of breath" as the rpms rise.

Does the motor rev until 3500 then stop and will not rev any higher (computer controlled)? Or does the motor rev up like its not getting any air, then just accelerates barely enough to pass 3500 rpms?

In addition, oil on the spark plugs is not good, but I'm thinking the problem has to do with the motor not getting enough air or not getting rid of it quick enough out the exhaust.
Is your timing off at all?
Clogged/dirty/wet air filter?
Obstruction in intake path (before filter, in resonator) like leaves, etc?
Obstruction in exhaust path (CC, bent pipe, etc)

I don't think its a loose vacuum line or a cracked line, as this would lead to TOO much air getting into the motor, which would be the opposite of what you are experiencing.