Ok, thank you Richard.
I'll try to get a freq meter and see what exactly going to the oscillator.

The chip I'm using is PIC 18F4685. What about to connect a 32768Hz watch quartz in order to feed the Timer1?
I've had no experience yet with this approach so I don't know yet what exactly I'm talking about.
All I need is to interrupt each exactly 100ms. It is possible to have that using the Timer1 feeded by an external source? And if so, using what setup?

Thanks in advance.
Regards,
Mike.