On my specialized I have a triple 105 setup. When I am out of the saddle and cranking hard and in the middle or big ring sometimes it seems like the chain disengages for just a split second, I don’t know how else to describe it. I was looking down when it happened once and it didn’t seem that he chain tried to slip over into a different gear on the cassette, it just looked pretty normal.
I know this is slightly vague, but does anyone have an idea of what might be causing this?
Thanks

How many miles on the drivetrain? Have you replaced either the chain or cassette without switching the other? (even just a little more wear on one vs. the other can cause skipping when under load)

I have seen guys change to a new freewheel, with a relatively new chain on race day. In testing on the trainer and in the parking lot warm up, no problems, but under race load it would skip.

Chains wear out, they need to be replaced, take it to the LBS and have them measure how much it has stretched.

this phantom skip happens to me from time to time. i turn the barrell on the rear derailleaur when it does happens and it fixes it.