New to this too, so shooting in the dark here...

If it's under-extruding sporadically like that then maybe friction somewhere. Possibly in the bowden tube (maybe strings of filament from a change at some point, or a kink in the path), or in the filament coming off the spool (tangled or overlapping on the spool, or getting caught up while pulling off because of a non-direct path).

Because it's happening more frequently when you're staring a new line, it could be in your retraction settings (maybe it's taking too long to get the proper amount of filament back to the nozzle after a retraction > bed movement > feed > new line.) I've found that retraction settings change pretty significantly between filament types, which might explain why you're just now seeing issues, especially if you are using new filament.