tiny84 .. strange "millis()" function results with interrupts

hi to all,

this is my loop: (tiny84 8MHz)
Sleepy::loseSomeTime(10000); // 10 sec, sleep
g_currentMillis = millis();

this is the ISR of my pin_change interrupt:
void ISR_wakeup_fototransistor()
if(digitalRead(opto_PIN)==1) g_flg_lampeggio=1;

when interrupts occurs rapidly the millis() are unpredictable ( increments few SECONDS each interrupts)

this works well if NO INTERRUPTS occours.

Any idea ???

thanks in advance
best regards to all.
Mauro IK1WVQ

Please use code tags. If you want help with a sketch, please include the whole sketch.

How is the ISR installed? In particular, is it edge or level triggered?