This is not surprising. My screen currently runs at 100 Hz vertical, 81.3 kHz horizontal, 1024x768, thus the maximum frequency carried is
1024 x 81.3 / 2 = 42 MHz.
2000 times the highest frequency required for a speaker cable !

At this frequency, the quality of the cable can make a difference. Moreover, I don't know anything about the ouput / input impedance of the connection, that may not benefit from a near-zero output impedance, like speaker cables.

81.3 kHz * 1024 * 1.33 * 3/2 = 166 MHz

* 81.3 kHz = Line frequency
* 1024 = Image has horizontally 1024 visible pixels
* 1.33 = a typical ratio between visible and total horizontal pixel is 1.30...1.35
* 3/2 = normally the 3rd harmonic should not be damped by more than 3 dB. The "3" comes from the 3rd harmonic, the "2" from the fact that the maximum base frequency is pixelclock / 2

When we're at it, I remember my new LCD looking clearly worse (somewhat horizontally blurred, noticeable with black text on white background) at 1280x1024 75Hz, compared to same resolution at 60Hz, using a VGA cable that came with it.
(then I bought separate DVI cable)

I also had voodoo2 passthrough (VGA) produce clearly noticeable blurring at 1024x768 and above, even when voodoo2 itself was inactive.

That hasn't much to do with cable quality I think.
The pixelclock in a TFT is optimized for 60Hz input - if you run it at 75, the screen can't keep up and some divider kicks in. The effect is that your screen will be running at some odd frequency below 60Hz.

Nope, blurring completely gone with DVI cable. Unless the TFT has problems receiving VGA signal at such frequency (or my videocard has problems sending).
I didn't bother comparing 60hz VGA vs 60hz DVI though, but I remember 70hz+ VGA being worse than 60hz VGA.
The effect was very similar to what I was experiencing with voodoo2 passthrough before (except it was much smaller than with voodoo2, and I noticed the voodoo2 effect on a CRT).
Anyway, the voodoo2 effect proves that extra obstacles on the way can significantly affect quality of VGA signal.

Yes, that's what I think. Getting a digital signal and changing its frequency is a different process than locking onto a non-native analogue signal and then getting a digital signal at another frequency from it, right? So perhaps the analogue circuitry in your TFT is less up to that task.

Similar experiences with my LCD. With some positions of the cable, the colors on the screen are b0rked.

I probably wouldn't give a damn even if it was some serious issue because (1) I couldn't find anything wrong at 60Hz, (2) there is no real reason to use refreshrate above that with TFT, (3) I use DVI cable now (I got it a few days after the monitor, had to buy it separately).
Anyway, blurring at 75Hz looked about exactly like the voodoo2 effect (same for 70Hz, but slightly smaller).