They might just not be suitable for you mate sorry to hear you are suffering- it will undoubtedly be a muscular imbalance somewhere in your body that the driving position in an e36 exacerbates
I had it once when I bought a saab 9-3. I sold it within 3 weeks because it upset my hip big time!
Get yourself booked in with a sports therapist and see I that can point out what's going on with your back
I bet it's your hip flexors or something getting tight and placing too much tension on the lower spine.