I have a 2000 Boxster S w/ Tiptronic. During most of the Autumn, Winter and Spring the engine runs smoothly. But when the weather gets over 75 degrees it misfires at Idle after warm up. After I get going it runs fine but sitting a traffic light it stutters and splutters. Took it to a shop last fall - said the 3rd cylinder was dropping out -- didn't/couldn't find out why. Ran well all winter but is doing it again in 90 degree heat wave here in Philly. Techron works OK - but next tank full it starts again. Am using Sunoco 94, too.