Test the thermal switch theory
If ten lights are failing at slightly different times, the thermal switch theory is a good one and you can diagnose that by pulling one of the lights down out of the ceiling and letting it dangle by its cable, or if it is too heavy to make that safe, by hanging it with string or a wire hanger, or disconnecting and replacing it with a simple light bulb pigtail socket. You ought to then see other lights fail but not this one. If that happens you know it's the thermal switches and you can either try to cool the attic, use lower wattage bulbs, or replace the fixtures.
Find the wiring problem
Convenient that the problem is in the attic where you have access to the junction boxes and cables.
If the previous test says "not thermal" (IE the light under test still shuts off even if it's not in the attic or if it's replaced with a simple bulb) or if all the lights are failing at exactly the same time, it's probably not thermal, at least not the thermal switches in the lights. You were already up in the attic and know where the cables are. Make a map of how they are connected and go back to the first junction box that ALL of them have in common, IE the point that feeds them all. Connect a power socket in that junction box, in parallel with the lights, and an extension cord going downstairs. Plug in a lamp and see if that fails when the rest do.
If the new lamp fails in the heat, you know the problem is between the switch and the junction box where it's connected. If it does not, you know the problem is in the wiring from there to the lights, and almost certainly in the piece of cable between there and the next junction box. Move your new power socket from junction box to junction box to narrow down the location of the fault.
You'll find the problem eventually this way.