losing plants to center rot.

[Steve Johnson]

I think at least one of them could be reverting to normal growth. Not sure why, but it does happen in cristate and monstrose cacti.

[mikethecactusguy]

I thought the same Steve and started to remove the growths in question. Then I realized if I waited they would crest. I almost removed the one that is growing wild on the one plant but as you can see it has started to crest.

[ohugal]

I hope you find the issue sooner than later. You've got a great thing going over there.
I've read in Anderson's The Cactus Family, he (Anderson) uses a shade cloth with partly aluminum strips to reflect some of the sun back. Thus also the heat.
Also, I've also read on two occasions, you can water your cacti in the evening when it's very hot. They can then take up the water before it evaporates. Never tried it myself though.
Good luck!