自问 Vs 自刎 in spoken chinese

Today I became aware of an usual mistake I made when speaking in chinese.

I don't know if it comes from my french background but I've a bad tendency to randomly put a second or a third tone after some words starting with a fourth tone.

Then 自白 (zìbái) and 自己 (zìjǐ) are not a problem but my 自问 (zìwèn) often sounds like 自刎 (zìwěn - cut one's own throat).

As "我自问" (and 让我想一想) is very useful to me, I already may have shocked quite a few people with this mistake.

Btw, I never had any problem with 请问 Vs 请吻 (see newbie lessons with Ken). The fourth tone comes naturally here.