It is only coincidence that mIRC's find operations were fast enough to keep up with your keyboard's repeat rate... and when mIRC hits an especially large gap between matches, your keyboard sends that keypress before it is needed.
1.) Manually press enter/ space only when the find dialog apears.
2.) Lower your keyboard's typomatic rate under Start > Settings > Control Panel > Keyboard.
(To Khaled:) Could you please make the Script Editor's Find dialog remember its position when moved, instead of constantly reappearing in the center of the Script Editor where it is most obstructive.