This is exactly the same issue that I had, except in my case the last 5 digits were flashing as opposed to just three in your case. I contacted Elksmaker by email at cs@eleksmaker.com and they were able to diagnose the fault to being an issue with the LED board under the first flashing set of digits.

To confirm the issue you just need to remove the bottom plate and bypass the first flashing board to see if the rest of the LED's past the initial flashing board return to normal.

They sent me a replacement board free of charge. I replaced the board and all the digits returned to their normal mode.

I could not fault them with this.

There is a video showing the assembly of the clock as one of the early posts in this forum:

http://forum.eleksmaker.com/topic/1812/elekstube-assembly-instructions-video

You just need to remove the 4 screws holding the bottom on. Remove the bottom plate carefully as the lead from the LED boards to the controller board mounted on the bottom is very short. Disconnect this lead and the bottom plate is now free.
Replace the controller board,.
You will then need to remove the two screws holding the bland end plate on as you substitute the replacement end with the mounted switch board on this end.

The clock accuracy definitely has improved. The clock appears to only have lost 2 seconds in the past three weeks since I installed the new board and reset the time.

As for the flickering issue, my clock was flickering constantly from the second digit through to the 6th digit making the time unreadable, only the first digit was flicker free. After discussions directly with cs@eleksmaker.com after sending them a video of what the display was doing, they isolated the issue to my second LED board. When this was bypassed, the clock returned to normal - though it looks funny with the second digit blank, and only displaying down to ten's of seconds. Once I get the replacement LED board, I will update on the result.

One thing I found that by using a different USB power supply, the flickering varied in frequency, but could not correct the problem. With your display only flickering between a certain value, you could try testing with a different USB power supply and see if that fixes your flickering issue.

Update: My replacement LED board arrived today and fixed my flickering problem. The clock has now been returned to its rainbow glory.

I received my new upgraded board kit today. Installed and it definitely fixed the clock speed issue. The clock accuracy definitely has improved. The clock appears to only have lost 2 seconds in the past three weeks since I installed the new board and reset the time.

Unfortunately this did not fix the current issue with my clock in that 5 out of the six displays constantly flicker so that the values are unreadable. The device did not flicker on the original power supply until 6 weeks ago. I have tried with 4 different USB power supplies and all give the same flickering result.

Update:
After troubleshooting with cs@eleksmaker.com directly I was able to isolate the flickering issue to the second LED board on my clock and they have organised shipping a replacement. Well done to them.

Update 2:
My replacement LED board arrived today and fixed my flickering problem. The clock has now been returned to its rainbow glory

Update 3:
After running with the new controller board for over 4 weeks, the clock has managed to only lose time by 2 seconds. This equates to 30 seconds over a year. Very happy with the improved accuracy of the new controller board.

I received my new upgraded board kit today.
Unfortunately this did not fix the current issue with my clock in that 5 out of the six displays constantly flicker so that the values are unreadable. With the old controller board, the flickering would stop and start intermittently, so not flicker all the time. This is still occurring with the replacement board upgrade as well. The device did not flicker on the original power supply until 6 weeks ago. I have tried with 4 different USB power supplies and all give the same flickering result.

Update:
After troubleshooting with cs@eleksmaker.com directly I was able to isolate the flickering issue to the second LED board on my clock and they have organised shipping a replacement. Well done to them.

The displayed list of 11 "Existing Addresses" aboove only shows if you have yet to create an account. Once you have created an account to place an order, the only address visible under your account is your address info. This does not appear in the list of available addresses once you have created the account and will continue to only show the 11 addresses in the above screen shot.

@EleksMaker-Z

Thank you
Order has now been placed

Glad to hear it. Will be resending my info shortly. Love the sound of the upgraded features as well.