Ford Focus 07 immobiliser issue?

Cork red

Hi

Wondering if any advice can be offered with the following issues with 07 focus.

2 weeks ago I was walking away from my car pressing lock button which didn't work - I sat back in car and passive key not detected. I pressed lock button in car itself and car started . This happened 3 times that day before aa was called out and my battery was in need of replacing . I changed the battery and key worked fine for one week without any issues until I stopped to get petrol and again the lock button wouldn't work - 5 mins of pressing the immobiliser and lock key internally but no joy . I then opened and closed my door and shut it quite hard and the car started . This has happened numerous times and it's always slamming a door shut (not just drivers)that will recognise the key and start the engine .

This issue isn't happening every time . It's now and again which is very frustrating. The only thing I had done to the car is having a new radio antenna fitted as my other one snapped in the car wash -couid this be causing the issue ?

I have replaced car battery and battery in the immobiliser -

Thanks

show me the money.1

I would say your clock are starting to go faulty very common in that shape focus.