Update: it was a bad ground (specifically, rust in the socket). It wasn't where the bulb was connecting to the socket (although that was rusty), it was where the black ground wire connected to the metal piece in the socket. Cleaned up the rust and put some electrical contact grease on, and now it works well. This was on the rear left signal (the most problematic), and I'm guessing the intermittent issue on the other is for a similar reason.
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