I don't know how much money you are willing to through at this problem but I would strongly suggest that you buy some scaning software or scanning device to begin with. You will eliminate all of this guess work. Start logging your daily driving as well as timed runs. Also for the timed runs I use a G-Timer, not a stop watch, however it is no longer mfg. You could try the Gtech
www.gtechpro.com. If you are intent on solving this on your own and not involving a tuning shop then this is the way to go. Start with the diagnosing equipment mentioned then fix with tuning software or take it to a tuner. I had a very similar problem with my Vette and I was able to not only correct it but get the car screaming through the gears.