Yamaha DTXpress/DTXplorer/DTXtreme

Hi everyone

Been away - but now I'm back. For those of you who don't remember me, 
I was the original creator of the DTXpressions website and author of 
DTXChange. Still a crap drummer, though! Good to see this forum is 
still going strong after all this time and recognise some of the 
names from WAY back.
Still got the original DTXpress (had it for 4 years) and it still 
does what I need. Can't afford to upgrade anyway :)

Anyway, I have a question:
I noticed at a gig the other night that whilst playing 16th notes on 
snare and ride together, sometimes the snare would not trigger; 
sometimes the ride. When I got home I tested this and - yes - when 
playing any 2 pads at EXACTLY the same time, one of the sounds would 
occasionally not trigger.
Now - I can't recall whether this has always happened and I just 
didn't notice or if it's started happening recently, but it's really 
annoying.
Pad/Self-rejection settings are fine; the pads play OK on their own - 
but when 2 are played together, I get these weird dropouts.

Any ideas?