To be clear, the "previous" frame is the currently rendered frame (that has already displayed at least once), whereas the "next" frame is the currently rendering frame, which, once complete and delivered, then becomes the new "previous" frame, repeat.
With G-SYNC and no sync, the current max physical refresh rate is decoupled from frametime (when the framerate is within the refresh rate for the former, and all the time for the latter), so lowering your max physical refresh rate won't help frametime spikes in those scenarios (in fact it may hurt things as now the scanout time is slower, meaning the time between each frame's delivery is less frequent), only lowering your graphical settings and/or average framerate target might, assuming the spikes in question are system/engine-side and due to your system not being able to consistently delivery the average frametime of your current average framerate target.
Again though, if the frametime spikes you are experiencing are due to general I/O bottlenecks at the game engine-level (or other engine-specific quirks), lowering your graphical settings and/or average framerate target may only reduce the perceived length of each spike, and not so much the frequency, and only the developers can fully address that (and most don't, or don't ever fully over the course of their support of the game post-launch).