That almost certainly sounds like a case of VSYNC limiting your FPS to half sync (30) because SweetFX takes just enough resources to drop your system below 60.
Even if you get 58 fps, VSYNC on a normal monitor will limit it to 30 to avoid desynchronizing.
3 solutions to this:
1. Turn off VSYNC completely, and live with screen tearing.
2. Turn on adaptive VSYNC if your gpu control panel has the option for it. This enables VSYNC when you hit 60 fps, and disables it when you drop below so you aren't capped to 30. Might not work with some games.
3. Reduce your game settings and/or intensive sweetfx settings used to reach 60 once again.