Is it neccessary to create a single surface for a mechanical/non-aesthetic part? If not, I would use a solid modeling approach with radii.
You just need a little more topology, loft the straight edges together and then use fill surfaces for the transitions. You will find it does a better job at capturing shapes than one surface which just gets dented and distorted, it's just what happens and that's true for any software.
One thing that might help out is splitting this into more features--don't try to form all the geometry with a single loft. Start with the simpler areas that are more likely to succeed, and then move to the corners and other problem areas after the primary areas have been built. (You might have to knit your surfaces together as you go to use features like Surface Fills in the corners.) Split it into regions, and it should work well for you.
I agree with Mark. Try and keep things simple and use easily definable geometry whenever possible. Lofts, sweeps and swoopy surfaces have their place, but if the shape you need can be produced with extrudes and/or revolves of clearly defined profiles, finished off with some drafts and fillets, use them. Unless you have a reason to make the part with as few features as possible (a bet maybe?) break it down into simple, easily defined, steps. I have a part that's made with a series of sweeps, just 8 features total, about 14 MB file size and takes 23 sec's to rebuild. A nearly identical version made with extrudes and patterned revolves, has 25 features, but is only about 5 MB, and rebuilds in just 3 sec's. In a big assembly that sort of difference starts to add up. Also, I'm guessing that this part will be cast at some point? If so, I'd bet that the folks who have to make your pattern will be happier with nice straightforward geometry to import and program. For that matter, if you have to make a drawing of this part (right now you are modeling it) you'll have a much easier time with dimensioning if you avoid hard to define shapes now. If you can, save the fancy surfacing for when it's time to make a fancy cover for whatever this sump goes on.
That said, if you really need to make this part with all the curvy surfaces, you might want to try using the boundary feature (surface, boss or cut). I haven't used it myself (admittedly I'm looking for the first good excuse to come along), but it's supposed to be quite superior to regular lofting for many things.
Thanks for the advice everyone, I will give these suggestions a try and let you know how it works out.
I mucked around with splitting up the surface again for a little while but had already tried that and didn't progress any further. So I took Mark and Erik's advice and tried to model it using solids. It worked pretty successfully I think.
Good point about the simplified design for pattern making too as you are correct in assuming it will be cast.
I have attached a few pics of how it turned out if anyone is interested.
Thanks again for your help.