I get too hot.

I have a problem with all the Matresses we own, i get way too hot. I tend to fall asleep fine but wake an hour later red hot. Once awake i can never seem to drift off again. It has gotten to the stage where i sleep on the sofa most nights and suffer from back pain. This has only come around in recent years, years ago we had a cheap open spring matress and i didnt have these problems. Anything with memory foam just doesnt work. What matress gives the coolest experience? I am around 13.5 stone and my wife is 10.5, we both also have lower back pain so something orthopedic and our budget is what the right matress costs.