2002 GMC Savana Miss Fire code

I have had a misfire code for some time only when I am accelerating and/or under load. Now the misfire code flashes whenever I pass 60 mph. I have never felt the engine running rough but a few times, but more recently I have. After an oil change it seems to stop the misfire until I am within the last 1000 miles of the 3000 between changes. I switched to full synthetic oil which seemed to stop it from misfiring the first 2 changes, but the last 1000 miles it has flashed every time I pass 60. I have had a complete tune-up. When I had it checked at a GM dealer early on (out of warranty) they wanted to remove the cylinder and check for binding in the valve. Then possibly machining the valve stem and hone the guide. Very expensive. I was advised that if it wasn't running rough and it only was happening infrequently not to worry about it.
Any thoughts??