I would first double check your crank arms and pedals . They are the most common part to fail on all bikes. If the crank arms got loise before they may have rounded out the square shaft and now there may be play in them.
If they all look fine take the shroud off and check your drive belt. If it looks wornbor if there is a lot of black looking " dust" in that area I would replace it. Last would be to replace the main crank bearings ( leastlikely)