Tested the existing GFCI at another location and it worked fine. Thinking the only choices now are the breaker itself or start digging up the line. There was a shovel cut repair underground several years ago that may be the culprit. Anybody know what the best splice repair kit is? I last used a kit from the HD that was designed for underground applications.

If I had to choose between a breaker going bad and an underground splice going bad, my money would be on a deteriorated splice. Also, are you sure that: 1) wire rated for underground burial was used in the first place?, and 2) there isn't any hidden damage to the line other than at the splice?

Agreed. And... Yes underground wire rated. I installed it myself- and then nicked it with a shovel planting a small shrub about 10/15 years or more later! Not sure about other damage but half of it is in conduit. Think I'm going to pull that plant and see what it looks like under there.

The results: I cut open the old repair and I found the hot wire (black) was damaged- and possibly a choke point. It was just barely hanging on about 2 inches from the original splice connector. (I totally missed that when I first repaired the wire) So I snipped off all three wires at the black wire break point. I added a second connector, jumper wire, some black mastic/ insulation goop at each end (not shown) and a fresh shrink wrap. So far it's a victory!

Appreciate the report back. Glad that you resolved it.