Earthing on a 300tdi is,
Main one from transfer box to chassis & chassis to battery
The earth for the dash/guages etc is on the bulkhead under the bonnet, about 1/3 of the way from the drivers side, behind the airbox.
Final one centre of chassis for rear loom. (I never did find this one, I ended up putting a new earth form the rear lights on.)
Fix the earth first (which is causing the jump with lights on I would imagine) and see if it reads better then, mine sits dead on 1/2 unless towing up a big hill then it raises a bit, but not much.
Mine started running where yours is just before the rad blocked/was knackered, but like I said, sort the earthing out first.