The engine only appears to run better at higher rpms.
When was it last tuned up? Spark plugs etc?
An ignition wire or two could be marginal.
It could have a vacuum leak which allows unmetered air to dilute the air
/ fuel ratio; that will cause one or more cylinders to misfire,
especially at idle. at hot idle listen for a hissing noise near the
intake manifold. Look for a disconnected, cracked or broken vacuum hose.
Such limited use contributes to dirty fuel injectors. Add a can of
Techron in the fuel