I think "by the way" is closer to "кстати". I would translate "как раз"as "just". I just wanted to warn you. So something has already happened but the speaker anticipated it and wished to warn the other person but failed. So he makes an explanation (or an excuse) that he planned to warn him just now but this thing has already happenned.
- Ай! Я упал со стула!
- Да я хотел как раз тебя предупредить, что у него сломана ножка.

Да, кстати, "каждый раз, когда я наталкиваюсь..."