Why has my in-dash shift light started going mental?

Some days it doesn't tell me to shift, some days it tells me to shift at say a grand when I'm doing 30 in 4th...
All of a sudden. This is new.
Another example, I can bottom out 4th at about 900 rpm (Not that I should be, just as an example), and yet it is telling me to shift...

So what controls that light? What are/should the stock settings be on it?
I realize that you can run a grand or so above the recomended numbers, I'm just curious what my car is trying to tell me.

I'm running a '90 Vette L98 w/ WCT5

 

The shift light doesn't really operate at any definete points of data, but rather as a result of calculated data. Throttle position, vehicle speed, engine RPM, coolant and intake air temperatures, measured intake vacuum, calculated engien load, and the status of options (like A/C) will affect the operation points of the shift lamp.

Your example seems like a problem, however. Has anything been changed recently, like rear gears, a MAP sensor, etc?

 

Since you have changed quite a bit of stuff it may affect how the light works, but who needs a shift light anyway, I can tell when to shift WAY better than that stupid light, if I watch mine and try to use it during normal driving it doesn't come on untill I'm way past the rpm that the car needs to shift at.