Thanks for the replies.

I changed the o-ring seal at the back of the plenum.
The old one seemed a little compressed, but no major failure.
This did not solve the problem.

I changed the sparks plugs and installed a new air filter on 06/2003 at 86000 miles.

I experimented by disconnecting the battery to reset the computer code. After that it ran very poorly and improved over time to the usual poor state, and the CE light came back on.

I just had my smog check done on 02/2004 and it passed easily, so something happened all of a sudden.

I purchased my last mass air flow meter online, and I can't find the receipt (I'm usually good at keeping records) so it would be difficult to claim this is the true cause of the problem (downside on online purchases).

I might try the cleaning procedure.