It seems I'm not the only one having this problem, as I've seen different threads about it but no replies

I've got a Realtek 8180L based wi-fi card.
It runs smoothly under windows 98 and xp.
I've installed it under Debian using the same xp drivers and ndiswrapper.
But it seems to randomly turn off during reboot or even during normal usage.

How can I reset/turn on the wi-fi card under ndiswrapper and linux? I could put such a command in cron and have it fix my problem partly.

Or why is the card turning itself off?

What exactly do you mean by "turning itself off"? If wlan0 (or whatever interface you have configd for wireless) disappers and you have to restart from scratch (ie. reload the module, etc), I have no idea. But if iwconfig settings are "reset" to defaults and you lose your IP, I may be able to help...

Are you using a laptop? If the card has power management built in, I've heard that can cause problems. (Power management can be turned off with iwconfig...not exactly sure how, but it's in the man pages.)

In my case it wasn't the power settings, but it was due to a couple other things.
First, I was not giving the ndiswrapper module enough time to load. I figured this one out when I wrote a script to load the module and set up iwconfig. When I tried to set the iwconfig settings immediately after the modprobe ndiswrapper command, nothing would be set. But adding a "sleep 6" command after modprobe fixed it...you basically have to wait 6 seconds for the modprobe to fully execute.
The other possible problem is the order in which you're setting the various iwconfig settings. The essid has to be set LAST--I'm not sure why, but apparently the card rescans the AP when you set essid? Anyway, I set the key first and then the essid and the problem was solved...at least after waiting for 6 seconds. Hope this helps.

"turning itself off" - I mean that the wlan0 is still there, but iwconfig wlan0 shows that no essid is assigned and the signal power is -95dB (that is I believe "no signal").
I will try your suggestions and let you know if they help.