I am comparable to you size and weight, and always had this problem. It could be saddle position etc as some have said, but I'd put my money on it being the saddle shape/size itself . It is hard to find one that fits (I have tried about 10, of varying styles and prices) .. Softer isn't usually better either..
Generally I'd say you need one with a larger central cutout on it. The selle smp ones seem really good, the one I've settled with is basically the same shape as these but was a lot cheaper (it was about 80% cheaper) and I can now ride without stopping for 3 hours (about the longest I've gone without stepping off) and get no numbness or pain whatsoever.. I think the shape looks pretty cool also, but that's beside the point..
Your lbs might be able to offer advice on sizing etc of the ranges they stock, or even have some put aside that can be tried. Or you could look on
eBay and try different ones - at least this way if they're no good you can have the choice of reselling and probably won't lose a lot of money etc..
The saddle I use was bought from China and cost me around £15 and is called a
CLIMAX CX1366 Saddle. I bought a second which now adorns my sons bike which has basically become my commute bike as he never seems to use it anyway.. I will say that I bought this saddle to see if the selle smp shape worked for me, I'm so happy with it that I think I'm sold for life. Should I not be able to source another in the future I will shell out the extra for a "proper" selle smp..