Fans at Max Speed CPU Throttled to 1.7 GHz

Hello everyone!

I'm experiencing a serious issue with my 2011 27" iMac and can't seem to find a solution. Here are the main problems:

- All fans are running at maximum speed.

- The CPU is throttled down to 1.7 GHz.

- The GPU is operating at only 20% of its capacity.

- After waking up from sleep mode, everything works normally for about 10 seconds, but then the problems return.

Here's what I've already tried:

- Replaced the CPU with an i7.

- Replaced the logic board.

- Installed new RAM.

- Upgraded the SSD (from a Samsung 840 EVO 500GB to a Samsung 870 EVO 1TB).

- Installed an OWC In-line Digital Thermal Sensor specifically for my model, but it didn't resolve the issue.

- Disconnected the central HDD fan because it was very loud; however, reconnecting it didn't change anything.

- Reinstalled macOS High Sierra.

- Reset NVRAM/PRAM and SMC multiple times.

- Checked all cables and connections; the LCD Proximity sensor appears to be in perfect condition, but temperatures for Hard Drive Proximity and LCD Proximity are not displayed.

I'm using Macs Fan Control, Intel Power Gadget, and iStat for monitoring temperatures, but nothing changes: the CPU remains at 1.7 GHz even under load, and the GPU runs at minimal capacity.

Has anyone encountered a similar issue or have any suggestions on what else I can try? I would really appreciate any advice or help! Thank you!