Minecraft Clock runs out of order

I created a Minecraft clock with a repeating block and chain blocks in a row.

Commands in the right order:
Added Scoreboard : *Sneak - stat.sneakTime
*Countdown - dummy
/scoreboard players tag @a[score_Sneak_min=1,rx=-85,rxm=-90] add Spawn
/scoreboard players add @a[tag=Spawn] Countdown 1
/scoreboard players set @a[tag=!Spawn,score_Countdown_min=1] Countdown 0
/scoreboard players tag @a[tag=Spawn] remove Spawn
/scoreboard players set @a[score_Sneak_min=1] Sneak 0

So I thought if I sneak and look to the sky my countdown goes up and if i look away or stop sneaking the timer resets

But now if i'm at full screen or a window which fills the whole screen it doesn't work - the countdown only goes up to different numbers and then again to 0 without moving my player
When my window is small or at the half of the screen it works correctly
But then if my game runs a while at this map it fails agein

So I think the problem is that the game doesn't perfom this commands in the right order