Voltage drop when lights are on

[DJ SIRE]

I have a 1987 S10 Blazer and just recently, when whether the truck is on or not, when I turn on my lights my voltage drops on the dash and the guage will bounce, the gas guage drop about half ways from the actual level and the temperature gauge increases a quarter way higher. Any Ideas that may cause would be appreciated. I had the battery and alternator test to be good and replaced both positive and negative cables aready with no improvement