Maybe it is software related. I experienced two uncommanded descents in the very early days of the B757. (Summer 1983). Both happened in approximately the same geographical position during flights from Gatwick to Palma, half way into the two hour flight. Vnav was not approved or used during the first year of B757 operation. The A/T and altitude hold were engaged on the first incident, when the thrust levers retarded and the nose went down as in a normal descent. The "descent" was cancelled and we continued on the Palma. On the second incident the A/T had been disengaged because it was slow to react to speed changes and tended to over correct. Consequently the alt hold dropped out, but we didn't descend. Boeing suggested a fluke pairing of Tacan,VOR and IRS positions had fooled the FMC into thinking it had reached the TOD position.