I loaded your save, I don't think the fluctuating car market share for the city is a bug. Might be a little ambiguous, but this game has a notion of 'local competition' that isn't an AI corporation player that you can interact with (i.e. bankrupt, acquire, transfer tech, etc.), rather they provide i guess 'natural' competition that is always present in a city regardless if no other corporation is competing in that city.

Notice how Beta Corp is selling 15 cars (the other Beta Corp store is selling 2) but the total number of cars being sold in that city is 15++? Pretty sure that's local competition making up the difference that is represented by the white portion of the graph effectively. If you look at other cities with higher cars being sold, you see the same fluctuation but it isn't as drastic, more like a wobble.

Hello, When showing my competitors market share in "Carthage" the pie is jumping around showing their share. Starts at 50% or so then falls to 0 and just keeps doing it.

Also wanted to report a crash from the game when my screen saver turns on.
Crash