Both TiVoWeb and Turbonet cards have been known to cause issues.

TiVoWeb is most commonly the problem with the cause usually associated with the modules that link into the TiVo events. Another potential cause seems to be when your LAN router crashes, this can also kill your unit. Both of these issues have been the cause of some of my users problems and have been discussed in further detail in the "Tivo Web dies" thread. Unfortunately there is not much that can be done if the cause is your router crashing. If the fault lies with TiVoWeb then the "KeepTivoWebResponsive" guide may solve your problems.

Turbonet cards have also been known to cause issues. If your unit crashes frequently without you making use of TiVoWeb then this can usually be attributed to the Turbonet card. The first thing you may want to check is that the card is properly secured, you may also want to remove the card and clean the board connectors on your TiVo motherboard with some alcohol then reconnect the card and make sure it is properly pushed in. If this still causes problems then you may need to adjust the Turbonet timing value. This has been discussed in further detail here and here. The conclusion drawn in both cases was to set the timing value to 'failsafe', this can be done via 'nic_config_tivo'.