Totally the same in my area. As soon as it gets hot the upload drops to about 0.01 and the yellow light starts flashing. Loads of T2 errors on the upstream, and some lead to T3.
Come the morning it's all back and working fine. We had a technician yesterday who changed the hub (which was fine as it was a Hub 4) and that the readings to the cab were fine. He went to check but he couldn't get in the cabinet because he didn't have a key that worked (said he'd not seen the lock that was on it).
I contacted VM and their chat person was terrible. They said it needed a cable repull and we'd have to wait until the 18th. I explained that we'd had someone over and what they said, but they were insistent there was nothing wrong and it is a cabling issue.
Anyways, it's all fine now and is supposed to be cooler today. We'll see what happens.