It's all about the Software

Keeping it cool is the answer to drift!
I fitted a small 5V fan to the USB end blowing air through. The ADC heatsink is directly in line. ~5 minutes after starting my local beacon was indicated at 1296.799231MHz. An hour later just 80Hz lower.
The case is virtually at room temperature.
But the signal wavers, the carrier shows about 10Hz wide. My 666 is much better shows no variations.
Looking again at the Chinese seller's description the reference is a VCXO. "-It adopts KDS VCXO crystal oscillator with large vacuum and potentiometer to fine tune frequency offset. It is not a cheap ordinary crystal to do clock." I wonder if that is the reason for it's instability? Possible variations of the voltage feed?
This would not be noticed on HF but the 888 has been improved for use at VHF up.
73 Alan G4ZFQ

What I use:

Nice USB fan

Dual 80mm fan

I suggest the second link - there's a single fan option, these can be set L / M / h, at Low setting they do the job well.

NO6B wrote: ↑Sun Nov 29, 2020 12:07 am BTW my RX-888 is moving over 4 ppm at UHF during warmup, so I assume it doesn't use a TCXO, just a simple uncompensated XO.

I believe the latest firmware is more efficient, generates less heat.

If you can find some welded wire screen screw that to the open ends for EMI purposes. It will also help with dust collection.
{^_^}