i have simillar to you

Made an interesting discovery:

If I set Dell Power Manager - Power Management to Cool, the performance of the Laptop improves while gaming, while if it's set to Ultra Performance, the performance drops. Crazy, isn't it? Works the other way around.

Anyway, at least now games are more playable (not perfect, some FPS drops), but I still feel like my G3 15 3590 its not delivering all of its HARDWARE capacity due to SOFTWARE issues, which is sad.

I have exactly the same issue. As soon as I put it into high performance mode the cpu boosts but then as soon as I play a game it drops to like 2 ghz and below. Random FPS drops in all games.

Seems like they haven't improved the thermals in the newer model all that much, my 3779 has the same issues. First you want to check the fans for dust. I had to take out my heatsink in order to find they had been clogged with dust. It seems obvious but it happened to me despite blowing them out on a regular basis. That being said, you need to undervolt the CPU to get better performance. They just run too hot to get maximum performance out of them. The i7-8750 in mine runs stable and cooler at -0.100V to -0.150V. Try doing that with XTU and see what happens. You can also turn down the turbo boost time window and power max. I found this helpful since my system was trying to use turbo boost during gaming which quickly sends the machine into thermal throttling and dropping frames. Just these 2 tuning changes should be sufficient, but you can go further and change the core multipliers to get them to operate more stable and avoid throttling. Once you get the CPU tuned you can also tune the GPU by under volting it as well. The voltage profile curve tends to be a little bit higher than what you actually need. There are tons of videos on YT showing you how to do all of this so I won't go into detail. By far the easiest and biggest improvement you will see is by simply finding a stable under volt profile for your CPU using XTU. The rest of these tunes require more software (MSI Afterburner and Throttle Stop) 

Hello,

I am facing the exact same issue. CSGO starts off with 250-270 fps and then after 5 mins down to 70-120. Monitored temps using AWCC and they were pretty normal too 69-73. I really need a fix. Any updates on the solutions will be greatly appreciated

i haven't found fix but the only thing that i can do is carry on switching thermal profiles from cool to ultra performance when gaming which is temporary fix for like 5 mins. When idle the cpu boosts to 4 GHz and when gaming it drops to 2.2 GHz and i don't know why. Temps aren't too bad.????

Same. After 5 mins CPU starts to throttle and falls between 1.7GHz to 2GHz, with wattage stable at 15W.

Not able to maintain framrate in games. I felt like I was cheated by Dell.

Yep. Sell this notebook - that was my solution. Dell never again. I suggest you a Lenovo Legion - way better thermals.

Yea, mine has similar problems except that the CPU remains hot for quite a while after gaming at low graphics

Mine keeps on staying as overclocked and at high temperature even during normal usage besides the mentioned problems