Thanks to all for the feedback. The 036 curves do seem to be a place where the speed seems to oscillate very slightly. I also have a lot of switch sections in series that could be a contributor. I did add some Labelle heavy oil (can't remember the #) to the gears under the trucks on an S1. Then gave it a good run at 15 smph for a few laps. Low speed control was noticeably improved. Maybe it helps to loosen things up a little.
I have a Lionel Legacy GP9 and it has exceptional low speed control. I seem to remember reading that Lionel speed control technology is different than the tach tape+reader that MTH uses.