Mike has a good point ... the temp you feel at one point may be quite a bit different at another point in the run.
The source of the problem could very well be that improper connection at the start of the circuit.
IR guns have their uses ... but they are no substitute to proper installation. Shoving two wires into a single lug is a pretty basic error.
Otherwise ... to answer your question ... you start running into trouble at 75C ... even if the wires have a higher rating on their particular insulation. Why? Because the places where the wires terminate are often only rated at 75C.
