Chronic overheating problem in Inspiron 15 5570

Hi

I am re-posting this, because it seems my earlier post was mysteriously deleted or is as of now unavailable. I CANNOT access my post as I got a 404 error when I tried to edit it. If there is a double-post, that should explain it.
Luckily, I had copied it before posting. I hope this will not vanish!!!

Hi there

As I am typing this, some app is showing the CPU temp constantly moving 77°C - and above, sometimes to the 90°Cs, which is way too high, with only Firefox + 1 Window of MS Edge running. With more applications running simultaneously on an older and weaker Vostro 3558/Win 10, it would almost certainly be mostly well under 60°C, mostly in the 40°Cs.

I used to use the program AI. Image Matting to remove image backgrounds with no problem, though it would shoot to the early 90s fora  few moments. The notebook NEVER shutdown because of overheating.

The last couple of times, though, it does, though I noted it may work with temp. below 80° but then it suddenly shuts down, possibly because it briefly shoots to some critical temperature, though the times I have noted it, it was NOT IN THE  90s, where it was in the past for briefly periods without shutting down. It is frustrating seeing the notebook having temps that high at all, as the norm, even more the frequency of shutting down the last 2, 3 times I have used this critically important program. I have not done video rendering of late, but I fear it might be affected as well.

• My notebook, which I got used about 1.5 years ago and upgraded to Win 11 & 16GB RAM, all Windows & other updates, including available drivers are up-to-date.
• Dells own SupportAssist finds NO problem with the fan or other hardware + all updates installed
• I keep opening up the casing for different reasons and there is no dust issue, all openings are clean.
• The other day, I decided to try out replacing the thermal paste, which i hoped would lead to a noticeable change. I followed instructions shown in diverse online tutorials to do this, but unfortunately, it seems to have made no difference. It was my first time.
• There is NO malware (according to Windows Security, Malware Hunter, etc), no unnecessary programs running in the background

In fact although my battery is virtually dead and needs replacing, I have put off doing this, because the notebook is constantly too warm, which I think it will affect it is life. An unconnected issue is a recent failed battery replacement, which has made me wary. It seems the battery was defective, as all readings like health at 100%, etc are good, but in spite of 100% charge, it does not deliver any power and the notebook immediately shuts down on disconnecting power. 

What could be causing this constant too-warm atmosphere?? I have a feeling it is related to this Inspiron 15 5570.

Does anyone else have this issue?
What can I do to return temperatures to normal levels?

Thank you in advance