That is usually a byproduct of a minor vacuum leak. Look around for cracked elbows, loose lines, etc. In the 1998, don't forget to look at the vacuum lines at the fuel pressure regulator (on the subframe, between the engine and firewall on the passenger side).

Clean the cylinder heads? Not needed at the present time, good lean combustion will clean them out once the engine is running well.
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My back feels better when I sit in a Volvo seat