Intermittent Temperature Spikes

Vauxhall Corsa 2007 1.2L generally runs at temps of 90-100 degrees, but after about a 30 min drive the temp will creep up to 110 for about 20 seconds then down to 90 again, then stay at normal temp for a while before going up to 115 for 20 secs then down to 90. It continues this cycle of overheating for 20 secs then cooling down to 90 over and over, getting slightly hotter each time and with shorter gaps in between, until it reaches 125 and coolant starts boiling and I had to stop and let it cool down.

This is all from one ~1hr drive. Prior to this it had a head gasket problem that was “fixed” with K-Seal, and it drove for about 4 hours straight with no issues.

Checked that thermostat opens at 92 degrees by feeling pipes and watching temp, no visible coolant leaks.