Yamaha 40 hp o/b - running really rough

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Got a 1992 40 hp Yammie o/b that's been a pretty sweet machine for the last year and a half, at least up till last Sat. It ran as it's always done all morning. In the afternoon, it started to idle really rough and seemed to almost stall a couple of times while I had it running at WOT. The motor actually kicked up a couple of times and at first I thought I'd just hit a patch or two of thick grass and that was jerking the motor up... but seems like the motor was doing that by itself... and that was halfway making me think it was the lower unit, but really, the motor was idling so roughly it couldn't be just that.

Being incompetent when it comes to motors... I'm prob gonna have to take it to south austin marine but before I do that and shell out half a month's wages to get it fixed, I thought I'd try to get some quick opinions about what I might try to do to troubleshoot the problem...

Quick facts:

- The carb was rebuilt about a couple of years ago - professionally done by the previous owner
- I replaced the spark plugs and fuel filter about a month ago

Someone said it could be bad gas... but if that was true, wouldn't it have run rough all morning instead of just in the afternoon?

Thanks for any/all opinions/suggestions.