I take it you have twin Yammis. Check your cable bends at both the helm and the motor. Not less than an 8" radius if you don't have 33C Supremes. Check your throttle friction screws on your binnacle. Loosen up, maybe a bit.
Your boat is 8 years old. Do a thorough check of your whole shift and throttle system. Things, especially mechanical, do tend to tighten up over time.