Try to increase both timeouts (of avast into the Advanced tab of Internet Mail provider settings and of your email account, in Properties of the account, Advanced tab too). You can set both to 600, for instance. If you’re on a DSL connection (not dial-up) it shouldn’t be necessary a so long timeout anyway…
Do you use any spam killer?
Sometimes the spam killer enters in a ‘loop’ and consume all the timeout.
Shutdonw it and start it again.