Mk3 1.6d - 'Jolt' and temp drop/wobble. Dark coolant/Kseal.

Hello,

Had my 2014 Mk3 1.6TDCI with 102k for 6months or so. I shouldn't have bought it really, private seller was quite obviously lying about things in retrospect and I'm too trusting and was in need of a car - regardless, its been okay really but there's been a few things;

Engine was oily, had a garage replace a few common-to-leak seals, oil level has remained same since, which was an issue. I replaced all the usual filters and did a coolant flush as was dark - which unfortunately had the presence of K-seal (or similar) within it. 

Since I did the coolant flush (some months ago) the temperature gauge has been more fluctuate-y than it was before. It used to climb to the middle of the gauge and just stay there, like my cars before it. But now it climbs to near middle, then drops a notch, then climbs to middle and stays there on longer journeys but still moves around a bit - you could say its reflecting the coolant temp I guess, with the thermostat opening/closing but it never used to do that and so was playing on my mind.

But mainly on a recent motorway journey sat at 70mph, just over 2000revs after 30min or so there was a 'jolt/lurch' I felt, as if I'd hit a small but noticeable pothole followed by the temp gauge dropping off the bottom of gauge, under 60dgC. This obviously caused mild panic - 'has my radiator just exploded' sort of a panic but car felt fine, no warning lights and then the needle started climbing again and all seemed fine, although never got back the middle of the temp gauge. I check under bonnet and all seemed fine, although have noticed the new coolant getting darker over time.

What was that?

My current theory/hope was it started a dpf regen with the jolt being the fan kicking in and the temp drop was from thermostat opening letting sub-60dgC coolant past the coolant temp monitor but I'm open to other ideas.

Does my thermostat/temp gauge seem okay from the above too?

Also, ideas on why my coolant keep going dark? Related to Kseal that's still partially present? Leaky EGR cooler?

I'm obviously also worried why there was Kseal anyway, but there's no signs of 'mayo' in coolant or oil and their levels seem consistent, just hope the engine isn't being held together by a thin veneer of kseal!

Thanks,

Nathan