My Laptop goes to sleep mode abruptly unprompted

I have been using this laptop since 4 years now. This problem persists right from beginning that is when the laptop was even under warrantee i had gone to dell service center and they couldn’t trace the problem infact did not come across this problem “as they claimed". Lets say the laptop was fully charged in off mode. Since i always used Microsoft Excel, I have now observed that it goes in sleep mode without any notification on screen even when the laptop’s battery is 80% or 67%. Then i keep pressing the space bar key or even power key to turn back the screen on, once the screen is on I am prompted to insert my password etc and then I am back to my work, but its really annoying that the screen would just go blank on your face while you are working. I have also observed that this problem occurs when i would put my laptop on charging that is when the screen is on and i see that the battery might die and I put it on the charging, the laptop would again go in sleep mode. I keep saying sleep mode as i always hear the fan running. I have also noticed one thing that the laptop goes in sleep mode at specific percentage of the battery life shown on battery bar specially at 18% or 67% or 87 or so. I remember intially when this laptop was purchased I was given an option for free upgrade to Windows 11( I guess), during that time the problem wasn’t there but some problem occured with the Windows registration key and I had to roll back to Windows 8 (which I hate the most). Please advice the solution. I have replaced power socket on this laptop once. I have changed the keyboard twice till date. The hard disk was cloned and replaced with SSD as the old hard disk had exhausted its life and the laptop was running very slow. There was a problem with the power key button also which was some how repaired and its working fine now.The battery has also been replaced once with new genuine one and that rules out if the problem was with the battery. Its adaptor also has been changed so that rules out if its providing unstable power supply to charge battery or run laptop that its spoiling everything that is replaced.I have many times paired my laptop with my TV wirelessly and watched live sports etc this thing doesnt happen but it has been noted if we work on Excel strange !. Your solution will be of utmost help to my missery with such an annoying problem.