99 Tahoe 5.7L, 162K miles - SES light - EGR

My service engine soon light comes on intermittently. It will stay on for several days, then go off for a couple days. Sometimes goes on and off in the same day. Went to Autozone and they checked with a scanner - it said "insufficient EGR flow" (didn't give me a code number). Checked the resisitances across the two circuits per the Haynes manual (10 ohms coil, 365 Kohms sensor). Book says to replace EGR valve if either shows an open circuit, but it doesn't. Have checked a dozen times over the last week, engine hot, engine cold, and get essentially the same values. Checked the connector and found 12V at the coil terminals and 5V at teh sensor terminals, so I think the ECM is OK. Pulled the valve - plunger is free without alot of carbon buildup. Don't know if spending $150 for a new valve will solve the problem or if it is something else and that would be a waste of money. Truck has 162K miles (original owner, "over maintained" - it's mint and runs like a top). Has also become hard starting during recent months - requires several seconds of cranking before firing, but always starts and runs strong and smooth. When idling, you don't even know it's running. Don't know if this is part of the SES light issue or something altogether different. Any help? Thanks.