Agreed with Bob on two counts
1 - Overbore. The engine cylinder walls are just now too thin, transfer much more heat, and the cooling system cannot disperse it with not big amount of air flow.
2 - Heat dissipation - If the cooling system is compromised - old/clogged radiator - it will also continue the upward spiral of heat. Do a quick A-B comparison, buy a cheap radiator from e-bay/RockAuto, and see if it makes a difference.
And, agreed on the gauge - Why not get a cheap mechanical gauge, just toss it in, and see the temps more accurately displayed?
Heat has to be dispersed somehow.