Why does my car keep switching between 4th and 5th all the time?

I know I'm pumping a lot of boost into the car and it is an aggressive race build able to go up to 300mph+ (482kmh+) and in between 4th and 5th it keeps downshifting then up shifting like twice when accelerating. Any ideas what might be wrong? Here is my transmission tune for the gears, I'll include 3rd if that might be the problem too.

 

do you use arcade (automatic) transmission?

 

Yes. I do, but nearly all the time it's always shifting between the two on the track it's just more annoying seeing them shift into different gears. It's mostly 4th and 5th that I have seen, sometimes it drops to 3rd, but it's really annoying cause it tries to go and then I have to wait a moment for it to shift when it already should have been in gear, is there something wrong with my gear positions? I didn't completely tune them as I mostly tuned 6th because it's hard to tune the other gears and I believe if I tuned them too much, it would turn into a drag car maybe?

 

Nothing wrong with gear ratios, its just automatic gearbox that shifts when it redlines, you should use realistic gearbox and maybe also increase grip (better tires and less tire pressure)

 

I don't know what race tires are better? What rims are the good ones that give good tires? And the extra grip variable tires just give too much grip for any car, I don't see how they can be used with extreme power, I have race tires on my car though