Latitude D620 and slowdown issue

Hi, I have an annoying issue with my Latitude D620 and hope I could get some insight on what the problem could be and what are my fix options.

The laptop itself has performed miraculously well over a course of about a year, until it started getting really hot and gets occasional overheats + shutdowns. However, when I let the laptop cool and turn it back on, everything was back to normal.

Until recently, I let the laptop overheat itself twice in a row as I was trying to encode a video which I wanted to finish badly (CPU usage gets very very high while encoding videos).

Upon powering on the laptop now, it will start out at its fast normal speed, then after about half an hour to an hour of usage it suddenly begins to slow down tremendously. During this slowdown period, nothing is consuming the CPU, and even with all programs and tasks shut down the laptop still runs at a really slow speed. Even if I reboot the machine it won't return back to its original speed. Only if I shut down the laptop then power it up again will it start to run at full speed again, but the slow down process starts all over again.

I know it isn't a software issue as I had previously reformatted and reinstalled Windows with the same problem. I have tried fixes from reflashing the BIOS, setting different BIOS settings, reinstalling drivers/updating drivers, unplugging devices, but the issue remains. I also tried placing a fan directly near one of the openings and it didn't do anything, although it did reduce the CPU temp dramatically.

I also notice that when the laptop slows down, Intel Speedstep won't function anymore as it should, it won't throttle the speed when appropriate and stays stuck at 1Ghz.

I'm currently using some compressed air to blow out any dust inside the vents, but it doesn't seem like there's much collection in the machine. I was thinking of opening the machine to get all the dust out, but I am not really well bent up about taking electronics apart, especially a laptop.

What do you guys suggest? Do you think my CPU or motherboard is damaged from the constant overheatings? I had thought that the machine would shut down to prevent hardware damage, so is this possible? This is a pretty old laptop so it doesn't have an active warranty anymore.