Firstly apologies if there are a thousand threads on this topic but its like a needle in a around these here parts. So my Skyrim runs at 40 FPS outside and Upto 60 inside. However, any dungeon with that suspiciously baked in lighting effect - such as that round the bear in Helgen keep- reduce my frame rate to aound 15-20 fps. any fixes out there for this?

I think rewriting game engine will fix that. Cause on my not-so-weak machine I have the same problem

If its a measurable drop with a tool like fraps, then its either your CPU hitting a performance ceiling with the poorly optimized code its running (game engine) or your graphics cards are a hitting a similar ceiling.

to fix it
pick the low preset from skyrim launcher and retest the area you lagged in, if it stops lagging you have skyrims settings configured too high for your system, so you need to goto markarth and configure your settings so you can run a solid minimum of 60FPS around markarth in the daytime

in options on the skyrim launcher in the advanced button you have 2 tabs, ones distance sliders, more CPU/RAM related, the others eyecandy settings which are graphics card related

get your CPU/RAM (distance sliders) to run markarth at a smooth 60fps with ultra low graphics settings (isolates the GPU from the equation), when thats running sweet then increase the eyecandy settings in the other tab untill you see it drop under 60fps in FRAPs, then drop those eyecandy settings by 20% so you have headroom.