Chronic Backpain and Spasms......Help needed!!!

I treat backs only, coming accross mainly nasty grotty L/S and most of them find some improvement from a combination of a simple HEP with mobility, stretches and core stability work.
The back's that I can't seem to get a gripp on are the ones that go into spasm. They can get on well for long periods but if they do anything too physical, correct posture, stabilisers on etc, their Multifidus and other paravertebral extensors start firing, giving them their pain.
I get them to stretch a lot, work a lot in neutral and maily in saggital plane. Most of them already have some kind of musclerelaxant too.
Anybody experience with this, any advise here would be appreciated.
Thanks