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non-Audi...I need help passing emissions testing

This is a '91 Chevy 305 V8.

After two years of passing MA testing very much under the limit, it just failed
with 600+ gpm HC (limit 220) and 7.4 % CO (limit 1.2%) (I may have HC and CO
switched here.) 
Recently, the Check-engine light has come on a few times after about 1/2 hour
of interstate driving; code reads "EGR valve problem".
Is this the blatantly-obvious problem, or is several-times-the-limit emission
within the range of what a bottle of Techron, a new air filter, and any other
listers may have for this might be able to fix?
(BTW, oil change (Mobil 1) was done right before the test.)