What could cause an engine to not run smoothly at high speed?

A weak valve spring can indeed cause an engine to not run smoothly at high speeds. Valve springs are crucial components that help ensure that the valves open and close properly in synchronization with the engine's firing order. If the valve springs are weak, they may not provide enough pressure to keep the valves seated firmly when they should be closed. This can lead to valve bounce at high RPMs, causing misfiring or incomplete combustion, which results in rough engine operation.

While excessive fuel pressure, cold engine temperature, and a dirty air filter can also contribute to various engine performance issues, they are less likely to specifically result in the symptoms experienced at high speeds when compared to weak valve springs. Excessive fuel pressure might cause flooding or poor fuel atomization, cold engine temperature typically affects performance at startup or low speeds rather than high, and a dirty air filter generally impacts airflow rather than the mechanical operation of the valves at high RPMs.