Not an expert by any means, but OX springs really are nasty. I'd sooner put a standard factory spring in (can be had for around £8) than an ox.

As Alistair mentions above, the OX springs come in different sizes (from 1-7 iirc) from different groups of guns. Sounds like the wrong spring to me, though the OX springs always did have a lot of preload (again IIRC as we're talking 15 years back).