There are already several reports of this type of behaviour (search the forum).
You could try 2 possibilities (either one of them or both).
1:
Avast main GUI -> settings -> troubleshooting -> "load avast service only after other system services".
2:
Avast main GUI -> webshield -> EXPERT settings -> "...well known browsers only".
In each case, please REBOOT after changing any of those settings before testing.