I'm getting error code P0300 no other codes?

P0300 - Random multiple misfires. You say it doesn't show once the engine has been running but does misfire when it has sat. The description is textbook for a vacuum leak. At low speed or idle the engine generates more internal vacuum because the throttle plate is closed more. That will try to pull in more air from wherever it can because the pressures want to equalize. Now take a modern engine with vacuum lines and various pieces that are all in the system. A leak will do more at low speeds, which yours does, but you also say that it doesn't miss all the time. That indicates that the issue is also temperature dependent. That says it's something like an intake manifold or plenum leak that "seals up" as the engine warms up. There are a few methods to find them, one of the easier ones is to use carb cleaner or ether and on a cold engine start it and spray around the intake manifold. You are looking for a spot that suddenly changes the engine rpm. Then you hunt in that area. You can also do that on vacuum lines as well.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Valve cover won't usually do that unless it's a major leak and because it goes away it would mean the cover would be sealed. Instead, I would look at the other items first.