Hi... one thing after the next with my GS500... This time it was stood all day at work on the side stand in some torrential downpours for several points during the day and when I came to start it, the choke lever had no effect, then briefly it would work for a second or two and the revs would increase, and then nothing again, back to low revs below 1500 and stalled. I could move the lever fully open or fully closed several times and it had no effect on revs whatsoever. I had to hold the throttle open to keep the revs up whilst it warmed up, otherwise it would stall.

After being garaged for a day it seems to be back to 'normal'.

Any ideas what this is? I'm assuming water gets in somewhere maybe?

Maybe your airbox drain is clogged and a waterlogged airbox was causing the running issues? Hard to give a diagnosis for something like this if you're not able to reproduce the issue.

Maybe try using a spraybottle and spray water at different parts of the bike while it idles until you see the revs drop or the bike stalls.

Had a similar issue.. drove me nuts.. in my case it was a slightly loose spark plug - water ran down and pooled in the plug well and it caused all sorts of mischief.. The bike would run fine otherwise until it rained. 

Notable symptoms:
a) bike had been on side stand in the rain - would *just* start / run on choke - but die as soon as I tried getting some power.
b) slowing down after cruising along in the rain - bike would bog down then stop. 

Thanks both... that's a couple of things to try... spark plugs, and water spray.