If you tried absolutely everything then all hope is lost
Accelerate when? just at the start with a big grid or every induvidual car accelerating out of turns and such?
It might help to load a replay, run it, maybe fast forward, rewind and stuff so that it is allready in your memory before using Fraps at the same time.
With rfactor i used to use rfLite with nothing installed appart from the stuff i need and then max out all graphical settings. also replay fedility on high.
With fraps there are a few things to tweak as well, you could go for half-size (hardly noticeable difference imo but much less big files)
Also fps can be downgraded in fraps, could work as well.
Both a big help if you are looking smoothness.
And of cource when running both rf/fraps make sure anything else is shut down so the PC can focus on what you need.
Lastly there is rfactor merge with which you can re-arrange replays and cut out sections. I never used it for smoother framerates myself but i reckon a smaller replay uses less resources.
http://www.rfactorcentral.com/detail.cfm?ID=rfVCRmerge