What values does the script show for Tv, Av, Sv in each of the two modes ? Tripod mount the camera if you can, point it somewhere bright, press the "FUNC/SET" key, and write down what the script GUI shows right after that.
It's possible you are hitting some of the limits set by the @param values. Try raising the max Tv to 1/4000 for example?
Finally i got a bit of time for testing.
Reinstalled last CHDK 1.3 and scripts from scratch. Max Tv to 1/5000. Default and min Av to 2,8 (max to f/11). Other settings at default.
Tested again v.2.0 with a sunny landscape and camera on tripod (custom EV adj. to 0) i got these results:
In Tv mode i get on display a green EV meter at 1/640 F 3.0 ISO 100 (real exif are 1/640 F
8.0 ISO 100) = Very overexposed pic
In Av mode i get on display a green EV meter at 1/3200 F 3.0 ISO 100 (real exif are 1/2000 F
8.0 ISO 100) = Correctly exposed pic
In Sv mode i get on display a green EV meter at 1/640 F 3.0 ISO 100 (real exif are 1/640 F
8.0 ISO 100) = Very overexposed pic
Same test indoor with low lights no issues on Tv, Av, Sv mode.
Maybe a ND filter or diaphragm control problem running the script on ixus125 in bright light?
More. I found that, even if CHDKplus running in Av mode, changing my only 2 available f values (2.8 and 11) doesn't affect the ND filter, even checking exposure whit set key or half press.
The camera inserts it, despite of running script or choosen values, when i frame a bright scene.
I can ear a "click" at ND filter insertion. Confirmed visually by myself looking inside the lens.
Obviously there aren't exposure problems with this camera out of the scrpt.