Problems when only online? Eug.... what is your IRQ disrtibution like? Is it possible you have a network card or modem sharing with your video card? That would suck. Do you have DX 8.1 installed? If not, do that first.
Go to Start | Programs | Accessories | System Tools | System Information. Expand the Hardware Resources node on the left, and then select the Conflicts/Sharing node. See if anything shows up as a conflict. If not, just to be safe, check the IRQ node and paste your IRQ distribution into a message here so we can have a look.
Now, IF that is not the problem, there are some other things you can try:
Set your AGP apeture size to 128 in BIOS.
Gand get Detonator Destroyer and wipe out your current Dets.
http://www.guru3d.com/detonator-destroyer/ Then, try backing up to the 21.81 WHQL Detonators. Different sites seem to be of differing opinions on which ones are WHQL and which ones are not between the 21.81's and the 21.83's.
http://www.guru3d.com/files/detonator/ Go with the 21.81's with the big smiley faces, as I think that means folks have been having good luck with that version.
Most importantly... if you get a set of Detonators that work, DO NOT "upgrade" them. nVidia's later model detonators have been having some stability troubles from what I'm seeing here... so when you get a set that works, stick with them!