That is a very expensive "could be".
Did you check/clean the idle control valve?
Clean the throtle body?
I would keep searching before I spend $500.00
$105.00 is too much to change sparkplugs.
I would be suspicious of the mechanic.

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1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
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