To actually have avast allow TomTom, avast would have to be blocking it with an alert or message window. Then you would exclude the file name that is referred to in the alert or message.
Since you aren't getting either of those no one would know what to exclude. The other problem is if avast was blocking why would it wait until 75% to have an impact, surely it would happen immediately. That is what stumps me.
Deactivating all shields wasn't the greatest choice as you don't know what element of avast may have a conflict (though I don't have that issue) with. The most likely interaction would be with the web shield as all downloads would be scanned.
Disabling the web shield may the desired effect, or just disabling https scanning (or intelligent stream scanning) within the web shield settings, may resolve it.
It is hard to offer a suggestion when I didn't experience any problem when testing.