Same on G-CIVE at MIA at the weekend. Sauna conditions as we boarded and only started to cool after climb-out. We were told by cabin crew it was a ground power issue. Back at LHR it took a while to shut the engines down as the the 'generator in the back' had failed and they needed to establish ground power first. So not sure if it was the APU or ground hook ups or both that were dodgy.

I've been very fond of the 747s but might start to avoid them now, they're definitely showing their age.