You have to remember that the cooling system is not getting as much help with the warmer weather, as the rad is being blasted with a lot hotter air, so doesn't cool as quick.
It sounds totally normal, the bubbling is just localised boiling that's all and normal.
It is normal to physically see the tanks expand, they also crack eventually as a result of this.
So long as the fans are doing their job stop worrying, and don't forget going slower pottering about = less rpm less water pump speed = less water through rad = water staying longer in engine getting hotter, and despite staying longer in rad as a result the outside "cooling" air is not so cool.
tabetha