INT0 used as triggering

Dear collegues,

I'm trying to use the INT0 interrupt as trigger system. Basically I have in input a square wave having the low level of 8 msecs and I would like to have on the output a square wave having 10 msec or more.
The basically idea is to use INT0 to detect the high to low edge and synchronize on this edge the output. The output lenght is set by the timer4 counting msecs.

So, basically I do the following:

Enable interrupt INT0 (tried both edge or level triggering IT0/IT1)

Set TIMER4 to call interrupt every msec (tis works perfectly already)

Seems working but:

If I use edge triggering, the INT0 seems called by both the hi/lo and lo/hi transition of the input signal...

If I use level triggering, the INT0 takes fake callings during all period the input signal say at low level...

I'm getting crazy... anybody could give an advice? The processor is a Silabs C8051F125.

Thanks in advance for the suggestions.

Bye
Fabio