i recommend trying the following if you haven't already done so...
1.) format the card using the phone itself and not using a pc/pc operating system.
2.) if you're creating additional directories, avoid creating and burying your files several directories "deep".
3.) create directory names without white spaces. if you really need a whitespace, use an underscore or dash in its place, e.g. white_space, white-space, or whitespace.
4.) make sure none of the directories or file names have any "weird" or non-standard/special characters in them.
class 2 or 4 speeds should be fine for this and any other mobile/smartphones out there. at least, with today's phones anyway. i'm sure this will all change as technology permits. until then, paying for anything faster is a waste of money. the "class" speed rating pertains to "sustained write" speeds. one would should only really be concerned w/ faster writes if the memory card was being used with, say, HD video recording like that done w/ an actual video camera. the option to record video on a phone is just a convenience and not meant to be used for production-quality recordings. microsd cards used w/ phones are typically meant primarily for "read" storage, e.g. viewing/listening to music, pictures, misc videos, etc. in addition, a faster card *does not* mean faster performance. performance is ultimately dictated by the slowest component. if the memory is fast but the phone is slow, then performance is slow, and vice-versa. so, if you buy a class 10 card, it doesn't mean that you'll get class 10 performance. especially, if the phone was designed and engineered to go only as fast as maybe class 2 or 4.
i have a moto d2g and i'm in the market for a 32gb microsd card myself. so, i just happened to stumble across this post during my search. i tried looking for the read/write specs of the phone but, i still haven't found it. they don't put this info in the spec sheet. i'm not sure if this is intentional or not.
in any case, manufacturers will have you believe that buying and using a faster card will mean faster performance. in fact, they're counting on it. otherwise, how else will they make money off of the uninformed?