You're welcome in advance.
I had almost the same crazy thing happening to my lights. It drive me nuts for two years. I replaced bulbs, made sure they all were there correct ones, replaced the entire light housing with new contacts, checked wires for continuity.... Still couldn't figure out why the craziness was happening. I searched the entire internet for answers and one sunny day I started to see a pattern of people replacing their indicator switch on the steering column. In order to fix your problem you need to buy a new cruise control/indicator switch. For some reason this GM part fails over time.
If you still need proof, just read the comments from others that purchased the part on Amazon. You'll start to see a pattern much similar to yours. I did this on my 1998 2dr Tahoe.
APDTY Multi-Function Combination Switch with Turn Signal Wiper Washer Hazard Switch and Cruise Control for Models with Tilt Steering
https://www.amazon.com/dp/B00HS1HOFI/ref=cm_sw_r_other_apa_5VL4yb470AP5H
Part#
26036312, 26047330, 26054727, 26100985, D826A