Hello,
I don’t know a lot about Mario Kart 64 speedrunning so I apologize if this isn’t useful. I was playing the game via emulator in 2 player vs. mode. I played rainbow road and then switched courses to DK jungle Parkway. After switching I got a weird glitch were I was going about 2 times faster than usual. I was on 150cc when this happened. The glitch stayed throughout the time on DK jungle parkway even when replaying the course. The glitch only stayed on this course and any different courses just stayed at 150cc. I have no idea if this emulator only  as I don’t have real hardware to test on.
Hopefully this is slightly helpful.