my iPad won't connect to my iMac and iTunes via USB - charging bar on iPad flashes on and off

I have a 9.7" iPad purchased new in Oct. 2017. I used to be able to connect it to my iMac iTunes using USB to load music onto my iPad. I often had problems with the charging going on and off rapidly at the same time that Photos was loading on my iMac, caused by the connection. Usually the problem stopped after a short while and my iPad showed up on iTunes as connected. Later I saw an option on Photos to not load when I plugged in my iPad so I checked that to hopefully stop the "charging" problem. Now when I plug in, the charging bar goes on and off at a regular rate of about 1/2 to 1 second and the iPad never connects to iTunes. It happens whether or not the iPad screen is on (after entering my iPad password) or off, when I plug into my iMac. I use the USB connectors on the rear of the iMac (27-inch, Late 2013). I have High Sierra, 10.13.4, which was recently installed. My iTunes is up to date, 12.7.4.76. My iPad has IOS 11.3 (15E216).