199 KM/h and engine cutting out

I have this weird issue where my speedo can show 199Km/h while I’m actually doing like 50 and when it happens - the engine cuts out like on a rev limiter thing.

It also show some speed like 30kmh when I’m standing still.
I read somewhere it might have happened due to low battery voltage that fucked up the speedo or the ECU?

I wish I knew which one is it since they’re both so expensive to buy