The power adaptor that it came with was 130W. As it happens I did have another one around, but the results were the same with that.
I tried it with Speedstep disabled and the laptop will run a 30+ multipler with the charger plugged in. Would it do this if it had a damaged port or motherboard?
Disabling speedstep should cause the CPU to run as if on battery all the time. With each piece of evidence, it's becoming clearer you have a faulty system board.
Would this also explain why GPU-Z is reporting that the GPU is running 405MHz due to a power cap rather than the 666MHz it should be running at under load?
It depends on the "this" you're referring to. No, it can't be the disabling of speedstep -- that affects ONLY the CPU. If you have a power fault on the mainboard (i.e., a faulty power circuit or clock crystal), then yes, it could explain the slow CPU AND the slow GPU.
Since CPU failures are exceedingly rare vs. mainboard failures - and a CPU plus GPU failure even less likely, it's very likely that (assuming as above, the system DOES correctly identify the AC adapter -- you've never answered that question) -- that it's the mainboard that's faulty.
From my second post: "In the 'battery information' section, it says 'AC Adapter = 130W'. It is the original Dell power supply."
Is there some further information that shows if the system is identifying the power adaptor?
How long has it been behaving like this (assuming it was OK previously)?
Were any changes made to the system in terms of new hardware or applications installed?
Can you try booting to a Linux Live USB or CD and see if it's still the same?
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Just to update, I got hold of a 170W charger just out of curiosity and that fixed the issue. But surprised as the model shipped with a 130W charger. Is this still indicative of a fault?
One other issue I wanted to bring up: on Prime95 the CPU is quickly hitting over 90C and downclocking a bit (not massively). Is this how it's supposed to be?