Check the throttle cables, they may be a bit sticky and not returning immediately, when you start to roll off the throttle.
Just disconnect the cables at the throttle bodies end, roll the throttle on and off a few times, you will soon feel whether they are free or sticky (resistance)
If the are sticky/rough in their action, they may be gummed up (lubed and dried up gummy) or they may have a frayed area in the cable.
Either way, just replace both if there are ANY doubts at all.