Just to help the reason you new a cap to hold pressure is
per 1 psi water increases its boiling point 3.2 degrees F
So if your cap is 5 psi to low the drop in temp the water / coolant can reach is 15 psi making It boil sooner and over flow causing your symptom over heating,
Also when diagnosing try to not to get caught up trouble shooting the symptom try to trouble shoot whats causing that symptom
per 1 psi water increases its boiling point 3.2 degrees F
So if your cap is 5 psi to low the drop in temp the water / coolant can reach is 15 psi making It boil sooner and over flow causing your symptom over heating,
Also when diagnosing try to not to get caught up trouble shooting the symptom try to trouble shoot whats causing that symptom