The Turbo Bug has hidden away in CLASSIC GAME for over a decade now, and sometimes jams your speed up. I attempted many times to fix it but it seems to be a forever problem.
It really is just One Of Those Things, in that I could never get rid of it entirely. I believe it boils down to RPG Maker sometimes not being given enough time to run through a speed reset script before gameplay resumes, so it only occasionally neglects to correct your movement rate before the player resumes control, leaving you Very Fast Indeed.
CLASSIC GAME was a big learning project for me, so it is quite crude in its scripts when I go over them, but amazingly mostly works out alright.
Thanks for playing in any case, and if I could suggest only one thing, it would be to try dying somewhere else entirely and see if you're still fast on that last life.
I did learn to be very careful with speed changes since this issue came to light though.