ISUZU NPR 1990 DIESEL #1 CYLINDER INTAKE VALVE LEAKING

When I went to adjust the valves for the first time, the #1 intake was tight; I set at at .018, then about a thousand miles later, it was at .016 (all the other valves were .018) so I reset it to .018. #1
cylinder is practically dead at idle, but seems to be running at highway speeds. Anyone else have this problem, and is there hope that the valve will eventually seat? Thanks.
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