Valves really don't 'stick', they get so dirty that it won't close properly. The engine is designed to spin the valves at above 2000rpm, helping to keep it clean. Also, regular gasoline does not contain cleaning agents like the 89 and 91 octane fuels do. You might want to pour 2 Techron bottles of injector cleaner into a full tank. Or try some Seafoam cleaner treatment in the intake manifold to clean off the combustion chambers.

Before you get all caught up with ideas, get a compression reading on a warm engine. That will tell you a lot.
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