That is also a symptom of a camshaft lobe(intake lobe) going flat.If the carb suggestions don't solve your issue,you might want to pull the valve covers and watch the rockers as someone turns over the engine(without starting it as this can be messy).If a lobe is flat it should be apparent as the culprit will not rise as much as the rest do.I hope it's not the cam but it does happen.