I am hoping somebody can point me in the right direction to fixing this weird issue I'm having with 1 of the 4080s that I've got. I have given the card to my brother to run on PrimeGrid for me whilst I wait for an AIO for my other one because I'm planning to run dual GPUs. The only problem is that it's almost 50% slower than what it is when it's in my rig, for example running GFN18 on PrimeGrid it is only completing ~140/day whereas it should be completing ~250/day. I then look on GPU-Z, temps are within rang 50C with 60C hotspot but then I noticed it is only pulling ~170w when I should be looking at around ~250w.

I thought it could be the PSU cable extensions that are used so I ordered a bequiet 12vhpwr adapter for 40 series cards as the PSU is the Straight Power 11 1000w but this did not fix the problem. I then tried putting my other 4080 in it which I know runs fine in my rig and it did the same with that card. I then thought maybe the CPU (2700x) is actually a bottleneck which I thought wouldn't be the case as it doesn't use that much CPU power for the GFN tasks. So I override the CPU usage number to 1.5 CPU (BOINC was defaulting to 0.765 CPU for GFN18 using the app config xml and this brought the power up to the desired wattage (250w) but then it drops back down to 180w. I then change the CPU usage to 4 but then look in task manager and it is actually still using just 1 thread, not the 4 that I selected. I then choose to use Heaven benchmark to see if it reaches 250w and it does so I've located the problem to BOINC, is there anyone that can direct me to a fix?

Thanks in advance!

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First off, BOINC only detects your CPU and GPU, it does not use it. The project science applications use the hardware.
Second, the project science application sets how many resources to use. Memory, CPU cores, GPU. That you want an app to use 4 cores doesn't mean it will do so. Some projects do have multi-threaded apps, these will use multiple cores.

I'd start in the BIOS/UEFI to check if something there is set to go into low(er) power mode when hitting high power usage. If not in the BIOS/UEFI, then a third party CPU/GPU/power limiting app in Windows or in the Nvidia control panel.

If he is using Windows, power management plan is likely the culprit.

Thanks everybody for your suggestions, going off a post found on the PrimeGrid forum, what fixed this issue was disabling the "Hardware-Accelerated GPU Scheduling" setting in Display settings -> Graphics -> Change default graphic settings.

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Thanks for the progress report. Knew it had to be one or more of Windows settings.