Air carrier flight crew reported an unreliable airspeed event during departure climb. The crew diverted and landed uneventfully.

Narrative

I was the pilot monitoring on this flight. ATC assigned us an air speed of 280k on the climb which the first officer did open the speed window and selected 280; climbing out of 18;000 feet I noticed the airspeed start getting a little bit low so I pushed on the yoke forward which we started gaining airspeed again. Few seconds afterwards the airplane entered into a clean install at that point the first officer training kicked in and reacted quickly which he clicked the autopilot off and recovered. We had no warning whatsoever that the airplane was going to stall. No EICAS messages and the airspeed didn't even touch low speed band. It seemed like it was an unreliable air speed event. I concur with ATC to verify our altitude to make sure that what we are showing as well. Once everything has stabilized; I called dispatch and talked to maintenance control about the event and requested to call one of the systems instructors or check pilot to get their feedback about the event. Duty Pilot was able to get Person A on the phone when she was a huge help for us and made my decision easier to divert to ZZZ. We called ATC and asked him to change our destination and he asked why we have an unreliable airspeed event so we were rerouted to ZZZ.Day 1; we had an ADC problem which was caused by a bad AOA on the left side; Maintenance have checked it a few times and they were swapping the left side with the right side to see if it's a fault in the sensor or what was causing the problem. So; maintenance concurred that it is the left AOA sensor. The weather was very bad storming very heavy rain throughout the day; the night and the next day as well. Day 2 Maintenance was doing the work during heavy rain exposed to that weather and was able to install the new AOA sensor. I don't know if that has anything to do with this incident or not. with all that being said to be cautious before we even took off; I have pulled out the QRH and opened it to the in-flight performance section for unreliable air speed just in case; and I handed it to the relief officer; and I asked him to have the pitch and power setting for us ready in case we need it.Maybe next time when maintenance need to do work on sensitive materials maybe we are able to get a hangar space or wait until the weather clears up.