If you can find some...try BonaKemi DriFast stains made specifically for flooring. With pine work, I hit each step first with the natural stain and then only a minute later I quickly apply the darker color. The natural stain acts something like a wood conditioner and the pine seems to take the stain more evenly. Also, this stain is the best flooring stain product I've used. Hardly any splotching with most wood even when used straight out of the can.

Any wood will stain better if it is all consistently sanded smooth.

However...in my experience with pine or fir stairs that have been in service for many years...be advised that the 'splotchiness' you are seeing may be due to permanent cell damage in the wood from being stepped on repeatedly over the years. This 'damage' can extend deeply into the wood (1/8" is not uncommon) and much of this cannot be sanded out. Even if you think you've got a clean and ready surface, once you hit it with your finishing product the 'shadows' reappear. If the splotchiness or a shadow appears where feet would meet the step, then that is what the problem is.

In cases where cell damage is evident and is the cause of uneven staining, very careful application of additional stain to the lighter areas can blend it all together to look good. Takes time and patience.

Thanks, glennsfc. I will try to find that product and take one more swing at it. It's also possible the dark parts are cell damage...although there are few spots that are on the side edges that shouldn't be. But you've given me hope. :)