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Since you mentioned a tight gas cap; I would suspect either the oxygen sensor may be faulty or the fuel mixture is out of balance. I had a similar problem with my 01 car around 10k miles this past January. Turned out to be fuel problem with the weather change (North florida has 2 seasons: summer and January). Gasoline companies modify additives with climate change. The dealer had several cars (from all models) with Check Engine" warnings. They had to adjust the fuel mixture. The problem went away.
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