I don't know if its been mentioned before, but I beleive I know what causes the shifter issues.
Ive had my G25 since it first came out in Australia, and I was happy to get it as most of you were. Two weeks into using it, I was having issues with my shifter not selecting gears properly. For example, I would be shifting from 3rd to 4th and it would go into 2nd instead of 4th, or going from 3rd to second would go from 3rd to 4th, etc.
One week later, I was shifting when suddenly I only had 5th and 6th gear, no matter what position the shifter laid in (ie even reverse read as 6th). Since where I lived, there were not many g25's going around at the time, I decided to bite the bullet and try to fix the problem myself. I proceeded to take the shifter boot/cover off and discovered something shocking.....
As most know, the shifter uses a co-ordinate style key mapping system controlled by pots. These pots have a small plastic connector which 3 wires are connected to using a crimping system. What had happened is that during the crimp process, the wire had been squashed soo much that it has broken off from the plug. I stripped the wire back and made a temporary connection to the pin, and the shifter worked as normal.
I then removed the pin and carefully soldered the wire onto it, put it all back together again and continued to enjoy the g25 for what it is. Ive been using it for many months now, day in and day out without a single issue.
This could be what is happening to your shifters. Take appart the shifter at your own risk, your warranty may be void. I suggest only removing the shifter dust cover/boot and inspect for any loose wires before proceeding to take the shifter completely appart... IT MAY NOT WORK FOR YOU.