1990 Chevy Lumina Unexplained engine miss

For no reason, it will randomly "miss" at speed. I could be driving the 30 miles to work at 65 or 70, and 10, sometimes 15 miles, maybe only 5 miles into the drive it will miss. No abnormal noises, it just misses. Maybe 3 or 4 short misses in a mile or less, then normal. Randomly happens and nothing seems traceable. I've had the fuel filter and air filter changed and have burned 89 and 91 octane. I have also run 2 of the last 6 tanks of gas with STP dry gas to try and eliminate all water in the tank. We drive this car 300 to 350 miles per week and get 24 - 28 mpg. Some days it's perfect with no misses. Slow speed, 25-35 mph, it will ping some, but nothing unusual. It has been occurring, randomly for 3-4 months. I know very little about this engine and the f/i or the ignition. Got any clues?