The only thing I see missing from your tune-up regimen is replacment of the oxygen sensor. The OBD-I ECUs allowed a wide tolerance in O^2 sensor input before tripping a Check Engine Light (CEL). So just because your CEL isn't on, it doesn't necessarily mean the O^2 sensor is operating at peak efficiency.

For the record, I only got about 22-26 mpg in stop-n-go driving, so your numbers aren't that far off. If you don't have records of the oxygen sensor being replaced within the last 3 years or so, I'd spend the $30 and 20 minutes required to replace it.

Also, save your gas reciepts and keep track of your mileage via the trip meter. Our gas gauges are not accurate enough to be very helpful in tracking mpg fluctuations..