Subaru decides when to start and when not to

i have a 1998 subaru legacy outback. two months ago had the transmission rebuilt (by a transmission company) because it was slipping for quite some time. about 6 weeks ago my husband drove my car in the morning and then i went to drive it a few hours later and it was “dead” - wouldn’t turn over but the lights and radio still turned on. jumped it and it started. worked for a couple of weeks and then it happened again. jumped it and it worked. took it to the mechanic and then it just kept starting so they said they could not “diagnose” it - one other time jumping it would not start it. then it happened again tonight and jumping it started it. in all cases the lights and radio still turn on. the mechanics have tested the battery and say it is not the problem. they say that it needs to not start to “diagnose” it but it is very hard to create that situation in the shop. please, please offer any advice that you have. i am so frustrated and afraid to drive my car when at any moment it may not start. thank you!!! sara

By your post I gather this car has an automatic transmission and the starter motor is completely inoperative when the key is turned to the START position? (no click sound, no nothing)

If so, there may be an issue with the neutral safety switch. This is the device that allows the engine to start in park or neutral only.
The next time it does this shift into neutral and attempt a start or shift into neutral and then back into park before trying it.

If this problem only started after the transmission rebuild then it could be the switch is out of adjustment, has been banged around and damaged, has an iffy wire connector, etc.

Usually when I see a car that starts only when jumped it is because the battery cables are loose or really dirty. The jumper cables make a good enough contact with the cars cables and it starts. I can’t believe that nobody has checked these connections, but that’s what I would guess. If the starter or neutral safety switch were bad, it wouldn’t start with a jump.

The first thing you need to do is clean the battery connections to make sure they are making a good connection. Even though things like the radio are working the connections can still be bad to make the starter work which requires much more current than other electrical systems in the car. If that doesn’t help the situation then see if you can start it in neutral. If that works then the inhibit or safety switch needs to be checked. If you hear a fairly loud click when you try to start it then the starter solenoid contacts are worn. You are at about the right time for them to start failing. They can be replaced with new ones.