Wifi at boot but not with ifup/ifdown

[Staus]

I am trying to use my raspi as a lab temperature logger but uploading data to Google Docs drops out after 8-16 hours. I think the wifi connection is being dropped after this time, causing the error. I can connect to the University wifi on boot with no issues, but a cycle of

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sudo ifdown --force wlan0
sudo ifup --force wlan0

fails to connect and gives an error:

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wpa_supplicant: /sbin/wpa_supplicant daemon failed to start
run-parts: /etc/network/if-pre-up.d/wpasupplicant exited with return code 1
...

Occasionally waiting a while between the ifdown and ifup commands, or launching wicd-curses in between, will skip the error and sometimes connect. Any reason this would be the case? Or a suggestion on what to change to fix this?