Remote door lock playing up!

[Retired Kiwi]

I have a Subaru SVX (my second) on the sunny Gold Coast in Queensland, Australia. It is fitted with a factory keyless entry using an integrated key/remote. No alarm. There is an after maket immobiliser.
I can lock the doors manually using the key.
Some weeks ago the remote stopped working - I assumed battery in the remote control but the local dealer tested and said it was OK and i needed to bring in the car for checking.
Then the car battery gave up the ghost and I got it replaced. Last week I took the car to the dealer to get the remote system fixed. They said the remote batteries were faulty and replaced them After dong that (and I guess re-programming the remotes?) this new problem arose i.e. when you push the remote button the doors lock and immediately unlock!
The serviceman has no idea what could be causing it and said they would have to dismantle everything and test out all components and that this would be a very expensive exercise, possible about $2,000!
Has anyone experienced this? Any suggestions?
I suspect that I may have to ditch the factory system and go for a Brandt aftermarket system which would probably cost about $500 to give me keyless entry/central locking and another immobiliser!