To be honest, this is primary the reason why I never really caught on with stunting. It's just there's so many things affecting stunting, like your FPS or if you got FL on or off(talking about SA, since FL off already counts as modding in VC).
People with a better PC than mine usually landed stunts more easily on SA-MP, than I did. And talking with J-Fox about how "GTA SA physics" work, it's no wonder. Even your clothes affect things like max velocity, so stunting in underwear would actually be better.