My thought would be the magnesium. Often when the magnesium is low people have troubles with the Cal and Alk going all over the place, the common response is to raise the Mag to 1400 which helps stabilize everything. So what I'm wondering if you locked down your Mag at 1400 then the Cal and Alk are locked in at their relative positions. If you truly don't have any SPS left, then I might try to drop your Mag, then adjust your Cal and Alk to the point you want them to be at....then raise your Mag back and lock everything together.
Randy Holmes Farley is the king of aquarium chemistry. Half of what he says goes over my head, but if I sit down and read through it I sort of understand it a little better. Although I admit I'll never be as knowledgeable as he is. He states "Preventing
magnesium depletion is especially important in maintaining appropriate calcium and alkalinity in aquaria." That's what led me to my thought above. Here is one of his articles where I got that from:
http://www.advancedaquarist.com/2004/4/chemistry and here is another that I've found super useful when it comes to making corrections in the Mag, Alk, and Cal:
http://www.advancedaquarist.com/2002/11/chemistry
That second article should help a lot.
Do you test at the same time of day each time you test? Toward the end of your description there you say you tested 2 hours after dosing, just wondering if you test at the same time every time? The numbers you got from that test 2 hours after dosing are actually pretty close, 360, 8.7, and 1400....but I understand you are doing a large dose to get there. I really think that second article I attached should help a lot. Good luck, hope you get it back under control!