Random Shut Off as I'm Slowing Down

[Pickax]

Lately I've been having a problem with the GZ where it shuts off while I'm slowing down.

Sometimes it happens when I'm coming up to a red light and after I get it started again it will shut off as soon as I roll the throttle on at all even if I keep the clutch completely disengaged.

Other times, when I slow down to make a turn it will shut off if I downshift a gear or two.

It always seems to happen when I'm squeezing the clutch lever and after I downshift. As I said, sometimes it happens when I'm going all the way to first and other times it happens in a turn when I'm going to third or something.

As a note, I didn't notice this until after I changed the oil recently. I started using Castrol Syntec. It doesn't have any friction modifiers or energy conserving labels so there's no obvious sign that that's the issue but let me know if you know of any reason it could be. The bike is a 2005 and has about 1500 miles on it.

Thanks a lot for your help!