I've had a problem with lumocolor pens.  They're 
just not black enough for me, and that's when 
they've first put down, not gradually (they have 
a UV-protected guaranteed not to fade with 
sunlight seal on the bottom, so that can't be the 
problem) and to fix it I have to go over the 
lines a few times.  

What I've gone back to mostly is Deco Color Paint 
Pens.  They're a little on the thick side, but if 
you can learn to draw on the edge of the extra-
fine tip ones, you can get a really nice dark 
line, and I've never had any experience with them 
fading or lifting (how could they fade, they're 
paint, not ink) The only problem is they smudge 
easier, so you just have to watch where your hand 
goes--even if they do smudge, you can rub the 
smudges and even some of the lines away with a 
little antibiotic cream  (i read the ingredients, 
there's mineral oil in it that's what does it, 
but I don't know where to get mineral oil on its 
own), rub the cream off with a tissue, and redraw 
the messed up lines.

If its STILL dissolving/fading/lifting, you 
probably do need new paints.  I'm on Americana 
right now, they're very bright but a little goopy 
to dry, and Anita's acrylics, which dry smoother 
and come in a bigger variety of colors (but their 
blues aren't too great).  They're both craft 
paints, so you should be able to find one or the 
other at a local craft store, and pretty cheap 
too.

^_^