Hi,
Definitely U400-A is defect.
It has 100mV at least of offset, but it should have 100µV maximum offset per datasheet.
Its output at "B" drifts about 500pm, totally uncorrelated to the LM399H reference "C", which drifts 100ppm
These 100ppm drift are also much too high and lasts too long..
I assume that the input transistors of U400-A are broken , drawing several hundreds µA or mA of current, instead of specified 50pA, so affecting the LM399H output.
U400 is an AD706, which should still be available.
Before replacing it, please make sure, that nothing oscillates , neither noise on +15V supply of U400, nor its output..
U101-4 is drifting, because the processor detects some internal offsets and tries to compensate this by adding an offset, or something similar.. this may vanish also, if the reference circuitry is stable again, after replacing U400.
Frank