Exactly the same problem. 2.4GHz randomly disconnect on my 3B+. Works fine on my Pi3.
5GHz doesn't even connect, the 5GHz ssid is listed though.
Latest Raspian, clean install. (+ update && upgrade)
Temporary and annoying workaround. When booted to desktop, turn off wifi, and then turn it on again.
Although this only works for 2.4GHz. 5GHz still only shows ssids, but refuses to connect to them.
This issue is not just for one raspberry pi 3 b+. This problem is found on every RPi 3B+ we got, 5GHz just doesn't work, no matter what router, or OS on the Pi.
2.4GHz wifi is a flip a coin, some works flawlessly, other refuses, some has the dropout issue.
We have 20+ RPi3b+, and the fail rate and the oddities, has forced us to stop using RPi3+. (Going back to old RPi3, and thinkerboard etc. RPi3+ is just not an reliable product..)
Last edited by grynnan
on Sun Mar 24, 2019 8:42 am, edited 1 time in total.