Session Drummer 2 & 3 and Studio Instruments are all GM mapped as far as I know.  They MIGHT have some open/closed high hat differences which might need a map, but again, I've not delved into them enough to figure it out because I just don't ever use them so I don't have the time to spend on them.

But EZD, Superior, Addictive Drums, Battery, all the big ones have mapping extended beyond GM because GM doesn't cover enough mapping.  And of course they're not standard to each other, either, which I hate.  I wish GM would be updated to include all of the mapping that is extended beyond the original drum hits in the spec from the '70's.  then maybe at least all of the them would use the SAME map.  one map would be highly beneficial to all.

I suspect the way that Jamstix gets around this problem is with the outputs.  I would bet that all mapping is done internally and hits such as open/closed HH are routed internally to the correct output that you open in JS.  that way you wouldn't have to have a separate map.

 They MIGHT have some open/closed high hat differences which might need a map, but again, I've not delved into them enough to figure it out because I just don't ever use them so I don't have the time to spend on them. 

The OP really likes to keep us guessing as he's not able to give a clear picture of the situation, but this is my interpretation:
I understand the question so, that the OP is using EZDrummer with EZDrunmmer loops, as he doesn't mention anything else (and the title is "EZD issue"), and thus he has no need to map anything. If he hears "occasional snare", shouldn't the mapping be in order?? Or are there several keys used for snare in EZD?

If the hits are randomly skipped, the most likely solution is to double the buffer size in "MIDI buffers...prepare using xx ms buffers"