Thanks Colin.
My timers are working well.
I didn't use Roberto's or your routine. I simply use the code I wrote in my previous comment.
I check the timings by sending outp to the parallel port and saw good results on the oscilloscope.
In fact, it's a little complicated now: I have a first thread which launches 2 timers(interval=6sec) shifted by 2.5 sec, a 2nd thread to launch 2 other timers(interval=1.638 sec) shifted by 0.75 sec and a 3rd thread to record a log files every 100 msec. All is working together fine.