peeing outside of box getting worse

Hi there,
We've had one of our cats, Samantha, for about 13 years now. She was a stray who we took in. While we were at our old house, she developed a habit of pooping in our unfinished basement, despite the litterpan being right there. I don't blame her, it was icky down there. We moved to the new house about 9 years ago. She started doing the same thing in our finished basement, so we keep the door closed. It seems to have stopped that behaviour, even though sometimes she does beg to go down there (we're pretty sure she has to go when she does). No pooping problems since.
About 7 years ago she started peeing on the mat infront of our kitty litter in the sun room. We got a boot mat to catch the urine, and switched to a giant sized, open topped litter pan. Dad also made the commitment to let her out earlier in the mornings. It got her down to doing it only once a week. Since about Christmas though, things have gotten worse. She started peeing in front of the back door, so mom got a boot mat for there. We'd been noticing some wet patches in the living room for the last two weeks and today I caught her peeing in the middle of the floor...explains the wet patches.
We've been to the vet. She says it's not a UTI, and that we've dealt with it this long, we might as well put up with it for a few more years. Trust me, mom's not dealing with this for a few more years.
The litter pan was spotless, the mats were both clean and the other two cats (they've been part of the house for about a year) were napping in the next room when I caught her today.
We've tried non clumping, clumping (sented and unsented) and finally corn based litters in her pan.
We have a pan for each floor of the house, and the other cats very rarely use "her pan".
Her pan is in an area in a corner of the sun room which is not high traffic and she can see out in case of any approaching youngins.
I've read the peeing thread and don't see any extra ideas, although I might have missed something.
I'm at my wits end. Dad wont clean up after her (he's too lazy) and mom talks about putting her down quite frequently. You can imagine what this does for their calm conversations about it. Basically the job of making this better is in my hands, and I'm out of ideas.
please help.

Did they do bloodwork at the vet's?

You have more than one cat, are you sure it is her doing this?

If it is her, she has been doing it so long now that it will be difficult to break the habit.

Is she spayed? I assume she is?

Sometimes, even though you believe the litter pan is clean, to a cat's acute sense of smell it might not be. She may not like the location, or the litter you are using, really the list is endless.

She could just be upset about something and in her stress, she is just letting loose.

She has not had recent blood work, but did have some a couple years ago and the situation hasn't changed.

Yes, I'm sure it is her. We've caught her a number of times peeing on the mat. Usually it happens because we don't let her out quickly enough in the mornings. She'll poo in the pan, but not urinate.

She is spayed, and has been since she was of the right age.

Dad and I were talking about the possibility of getting a finer grain of clumping litter, and maybe adding another pan to the house. The problem with that is that she's so big that we can't put in convienent covered ones, they have to be huge and with bootmats infront of them incase she decides to do her business there. It will also have to go into one of the bedrooms.

She seems nervous about the litterpan. I don't know if it's because mom's yelled at her so much while there, or if it's because the little ones like to watch her when she's in there. Any suggestions on a better placement?

Thanks for any help you can offer.