The lock/unlock relays are very unlikely to be fail. You can disable the central locking system by unplugging a connector. That will stop your intermittent self-locking problem while you trace down the fault. The connector is above the driver's left knee, so you remove the felt/cardboard panel there and locate a three-wire white plastic connector. The wires going in-out will be Red, Green, and Black.

As others have said, the issue is probably in the system leading to the relays, and the most likely area is the door wiring or the switches. (Key and knob). You will have to pull the door panel and start inspecting in there. Another possibility is that the wires in the rubber bellows at the door hinge have lost their insulation and are touching each other. That's a place to look after the other mentioned places check OK.
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Bob: Son's XC70, my 83 240, 89 745 (V8) and XC60. Also '77 MGB and some old motorcycles