Arduino Mega 2560 Interrupt pin 19&20


I have been using the digital pins 2,3,18& 19 as external interrupt pins.
Pin 2&3 working fine but pin 18&19 doesn't work perfectly as expected (intermittent).

I have attached a WiFi shield to my Arduino Mega.
Is it possible to use pin 18&19 as interrupt pins even though i have already connected a WiFi shield on it?

Can anyone please advise on this?

Which wifi shield? There are dozens from different manufacturers.