Prevent voltage drops on Arduino
If the regulator cannot supply enough current then you will get intermittent problems. The voltage will sag when something attempts to draw more current than is available.
The solution in the link sounds reasonable - have you tried this? If not I would try it and if things still don't work then update things here.
The other thing you could try is to add a reservoir capacitor (say >10uF, preferably low ESR, or parallel a few to achieve this) across power and ground near the SD card (or whatever you think is causing the issue) 
This will only work if the average total current is less than the regulator can supply, which sounds possible. It will deal with any short demands for large currents, rather than the regulator.