Do you have a thermostatically controlled fan clutch? I'm not as familiar with the later 126s, but if the bimetallic spring is not operating properly to engage the fan at lower speeds it will definitely allow the heat to build up in stop and go driving, which would cause the rest of the symptoms you describe. You should verify the operation of aux fan(s) as well, before throwing parts at the problem. My sense is that it is not the cap but the fan clutch, but please let us know what you find out.
'76 Moto Guzzi 850 Le Mans