As a last resort, I pulled the crank pulley off to check the trigger wheel up close. To my surprise, I found that the wheel was loose.
The JB Weld had finally let go. I readjusted the wheel and added a nice, thick filet of JB Weld to resecure it.
After weeks of poor idling, the car now runs smooooothly again. On to air conditioning and EFI!