I've got the exact same situation on my 156 2.4 JTD and also had it on a 2.0 20v petrol Lancia Kappa.
Symptoms are nearly identical on both, checked on the 15 km trip from home to work: the temperature needle sits at 70 C on both, for 90% of the trip, slightly goes up on the Lancia if I catch a traffic jam. Just before getting home, I need to go up a pretty steep hill for about 2 km - I usually do that at around 80-100 km/h, which means 4th gear on both cars, 3000 rpm on the diesel, 4000 on the petrol - and by the time I get to the top of the hill the needle went all the way to 90C.
The water pump was supposedly changed on the 156 (confirmed verbally by the mechanic who did it, no invoices though and the mechanic is a friend of the previous owner's, so I'd take that with a pinch of salt) and was definitely changed (with invoice and everything) on the Lancia after I got it; I ran the Lancia for some 20k km without significant issues, so I'd tend to rule that out.
I did the test this morning for the head gasket failure - drove the 156 around the hill twice until it warmed up (to 70C), then went up the hill, temperature went to 90C. Took cap off, with the engine still running - no hissing, no bubbles, a slight amount of foam on top of the coolant (which definitely needs replacement), but didn't look like oil, as my oil turned black a few minutes after the exchange. There are no signs of "mayonnaise" in the oil filler cap, nor any bubbles on the dipstick, car seems to be running fine, so I'd (wish to) rule out a HGF too.
I guess this leaves only several options open - airlock bubble, thermostat stuck open and closing under high load, temperature sensor under-reading in normal conditions and waking up only when going up a hill (I fail to see any logic in that, but it's 'Italian electrics' - could be the increased amount of water circulating past it which kick it into life), malfunctioning radiator and/or a design fault (not getting enough airflow when going up a hill?).
Not being an expert by any means, I'd really appreciate some other thoughts on the above.