Does it happen when it's parked on the center stand?
I tend not to put it on the centre stand because where i park it outside my work the ground is on a very slight uphill slope, so it can be a bit of an effort putting the bike back down.
I was wondering if the slope might have anything to do with things, but i've had the bike about 9 years and mainly parked in the same spot and this problem has been on and off only since last summer.
I can try it on the centre stand tomorrow though.
In the mornings i wheel it out of the garage, use the front brake to bounce the bike a couple of times and it starts better than when it's outside work for the day.
I thought the winter weather wasn't helping matters but today there were no excuses with that as it was nice and warm outside.