I'd start by tightening up your motions on your weave. The undercut is coming from not pausing long enough to fill and traveling to far forward with each cycle. Seems I remember having this conversation before with you possibly.
Try traveling only 1/2 to 1/3 as far forward with each side to side motion. You'll need to speed up a bit to get things to even out some. Again I don't think you are quite able to read the puddle yet and so aren't actually able to react to what is happening.
What he said /\
When I first started welding and couldn't yet read the puddle yet, I had instructor tell me to count out the hold on the edges. Your instinct is to pull away and move too quickly. Do a pass where you hold a long 2 count on each side all the way up, no matter what you think you're feeling or seeing.
Letting your angle go nuts can hurt too. With your side to side you wanna swivel a bit as you're swinging through the middle so your rod is pointing "at" the toe line on each side. But I'd concentrate on getting consistency on the timing first.
