6 Year Old Plaster Started Pitting

My pool surface has slowly started pitting over the last 1-2 years and I'm not sure why. It's been a very slow, gradual process, but now it has become noticeable, especially in the shallow end and partiularly all over the steps. You can't really feel the the pits when you walk on it, but they are noticeable because I had an algae bloom recently and the dead algae is stuck in all the pits and I can't seem to get it out with a stiff brush because most pits are small but deep. It affects mostly the floor surfaces, but I also notice it chipping away at the plaster on the wall surfaces too, in some areas. The grout in the pool tile is fine, no problems.

Background on my pool:
Located in Atlanta, it's about 24k gallons, 15x38 gunite pool, orginally built in 1953. I had it totally renovated in 2002, including acid wash, new steps, new plaster (with some blue quartz-like flecks in it), and new tile/grout, new plumbing, filter, heat pump, polaris 280....the works. I had problems with the builder I used, and don't really trust the quality of all his work, but the plaster job seemed like it was OK.

I was very good about brushing constantly after the plaster job and ended up with a baby-smooth pool surface by 2003. I never close my pool, and am pretty good at keeping things in balance (though I don't check chemical levels much over the winter). I use the borax/baking soda/bleach method of chemicals, plus polyquat occasionally. Only real problems I've had in the past (2-3 years ago) were CH that was too high (around 400) and some precipitating metal stains. However, both of those situations were solved at that time (quit using cal hypo and switched to bleach; and used vitamin C for metal pblm). Last couple of years have been basically problem free, with good chemistry, yet that's when I've started noticing the pitting problem. I will admit that my one downfall on maintenance is that I rarely brush anymore, unless I get some algae, which isn't that often. I rely on my Polaris to clean the walls & floor (though it doesn't get the steps at all).

Most recent readouts:
TC: 3.0
CC: 0
FC: 3.0
pH: 7.2 (occasionally rises to 7.4 at the highest and never lower than 7.2)
Alk: 85 (usually between 85-95)
CH: 220 (very consistent this year, but last year was closer to 250)
cya: 50 (last year got up to 80 at one point)
Temp: 86 (usually 86-89)

These test numbers hardly ever vary -- it's been quite consistent. I hope this provides enough info to trouble-shoot this problem.

I would like to know:
a) What can I do to stop the pitting from advancing further?
b) Is there any repair I can do to fill in the little pits so they don't collect dirt (mostly on the stair surfaces & edges)?
c) Would switching to a robotic cleaner possibly help keep the pits clean?

Thanks to anyone who can offer suggestions. I love this forum and have learned so much from it.
Ryan