Thanks for that definitive information!
Last night and this morning, I measured the 20 MHz oscillator. It's running at 20,005,717 kHz at the beginning of the transmission, dropping to 20,005.414 at the end of the transmission. Then it shifts up to 20,005.419. It then drifts down to 20,005.418, turns around and drifts up to 20,005.423. Then it shifts back down to 20,005.717 when TX starts. Summarily, the heat causes downward drift on the frequency and being in TX mode shifts it down by 5 Hz. I'm guessing that the supply voltage drops in TX mode, causing the 5 Hz shift. I've also seen about 1 Hz drop in frequency when the display backlight is on - also presumably a supply voltage drop.
Looking at the characteristics of the drift during TX, I'm guessing that adding the second transmission to the cycle would drop the average frequency by about 4 Hz. That's 0.2 ppm and would change the timing drift by 1 second in about 58 days if I calculated that right. I'm seeing much more than that.
So...anyone have other ideas on what could cause this?
FYI, the average measured frequency is 20,005.718. The setpoint I determined experimentally and entered into the U3S many months ago is 20,005.470. The 248 Hz difference is 12.4 ppm, corresponding to a drift on 1 second per day. I'll enter that in to the U3S now and see what happens.
But before I go, I have a related question. I have found that every time I go into the menus and change something other than the time, I seem to lose about a half a second on the internal clock. Is anyone else seeing this? Is there a programmatic reason for this?