appear to lost my HRC after tempering D2

Simple shaped parts, 7" dia. cylinder with 3.5 thru hole, 3.75 long. Wrapped and heat treated, 1200 for ten minutes, then 1850 soak for 3.5 hours. Allowed to cool down to handling temp, I checked the hardness after air cooling, 59 HRC average, right on what I was shooting for. Then tempered at 960 for 4 hours, with the plan of completely cooling down, then second temper at 900. I did a hardness check and now I am getting about 40 HRC, which will not do at all.
I have been religiously following the instructions in William Bryson's book, "Heat treatment, selection, and application of tool steels" for a couple years now and have had pretty good success, but am now at a loss.
Is it possible that I have a softened layer at the surface? I can set these parts up in the lathe and turn a bit off, as they do have stock on them. I just dont want to cut too much, just in case I have to send these out to be "fixed" and heat treated again. Any ideas on what is going on? I'm all ears.

Your soak time is way too long. You base it on the thinnest section (page 35 ;-) ), which in your example is the 1.75" wall. And it should come out much harder than that after quench. Possible you've got something other than D-2? I got some 'A-2' that came out of heat treat at about 45RC. Heat treater said his best guess was it was O-1. Said he sees it almost every day.

Might try a skim cut on one to see what the hardness looks like underneath. And some air hardening steels won't really harden fully in large cross sections without an oil quench. I don't think yours should be a problem, but it might be an advantage to pull it from the foil immediately so air can flow through the center. Probably not the first thing I'd try, but if I wasn't seeing 60+ before tempering I'd give it a shot.

Ooops, I did forget to divide! But would a longer soak show up in temper? I got this from a well known dealer, it acted like D-2 during machining. I guess I will try to check under the surface in a couple spots. I just dont see why I lost my 59, I would have thought 56 or so would have been the #...

Crucible says 600F for a 59 RC temper, although it says even 1000F temper will only draw it back to 55RC, don't know how it would drop as low as yours even with the extended time. I've forgotten parts in the oven at 350-400 over night before and not had any result like that.

There are 2 ways of heat treating D2 for ~60HRC

1) hardening temp 1850-1870°F / 1010-1030°C
temper at ~400-450°F/ 200-220°C

2) hardening temp 1930-1970°F / 1050-1070°C
temper at ~980-1000°F / 520-550°C

I think you mixed the two a little bit, but 40HRC is a little low.

Was your part clean of oil, scale, etc? Keep in mind 1HRC equals 2µm / 7,875*10^-5 inch