Help, laptop is slow when off charge

Dec 15, 2019
Okay, basically last year i got a new gaming laptop, Dell g3 15 with an 8th gen I5 cpu to be exact. I did get it so i could game but mostly it was for school, work, entertainment, etc. at first it was fine, i could have it on a flat surface, i could browse google, youtube, streaming sites, anything without uncomfortable heat. Then it came to gaming, a few months ago i wanted to game again,so i booted up a game, played for a few hours

bam, laptop began getting way hotter then it should of. (it was minecraft,usually around 30fps with high quality shaders, went anywhere from 60-98 degrees celsius)

that bugged me so i did some research, blah blah blah found out undervolting was supposedly safe and lowered temperatures.

so i did, i got to like -120v or something? using XTU, it worked great, except, now whenever my charger wasn't plugged in my laptop went slow, hot, crashing and this is when not playing games this is just having 1 google tab open with grammarly and Ublock origin running. so blah blah i get annoyed and reset it, change it to default voltage offset and so on, uninstal XTU and everything is fine right? wrong. now my laptop uses 40-60% memory when no apps are running and i'm just on idle or browsing youtube, overheat frequently when doing anything and when off charge it overheats, slows down and i have the constant risk of it crashing when i'm an hour into my class work and i lose it all.

soi tried resetting it completely, nope, tried adjusting setting nope. tried everything that was safe and nope.

now i just noticed when on charge the processes are like : CPU - 5-60% depending on if runtime broker is doing some random shit. Memory - 40-70% depending on what apps im using. Disk - always 0% and Gpu - 1-15% depending on what i'm dong

off charge it really stays the same except GPU, it jumps to 98%

so can anyone help? my warranty runs out in 2 weeks, my dad (he brought it for me) is trying to get the receipt or proof of purchase so we can take it back to JB Hi-Fi and get it repaired but i would desperately like to do whatever i can to fix it myself ,because either way i am going to get really annoyed and either ask to copy my important files over to a new factory set laptop, or get them to have this fixed. and if they don't then well have to take it to a repair place and just, i really need it fixed for my courses in 1 1/2 months

any advice appreciated, thank you.