If you are playing the or a modded version (like the NextGen Edition ), make sure your frame rate is stable. Some older GTA missions, particularly those involving RC vehicles like "Demolition Man" , can become significantly harder if the frame rate isn't capped properly. To help you further, could you tell me:
Send F8 ; Invisible hotkey bound to volume set SendRaw "streamvol 8" gta vice city all set stream volume-8
Enter .