Edge 305 giving inconsistent and inaccurate elevation readings. everything else is cool. any thoughts?

Same for me very often.
I think this is due to starting a road without waiting long enough that sattelites are acquired.

In fact 3 sattellites allow to compute position, 4 are necessary to compute position plus altitude.

Accuracy of position and altitude is much better from 5 sattellites acquired.

So I advise to start your device OUTDOORS, in an open place (not near walls as it masks sattellites) and wait until screen automatically moves to run mode, meaning enough sattellites are acquired.
You will get better chances to have a good accuracy.

Also on Garmin connect site, when you load your ride, you can correct altitudes automatically using real maps.