Instant pace erratic at best

Have been running a 10k pb plan for me over the last 5 weeks now. Yaaaay I hear you cry!
Anyway, in attempting to monitor my pace instantly I have recently included a pace screen into my run app. Ummmm its bloody woeful. Booo I hear you cry
It jumps around by a minute or so for my instant lap pace. This renders it useless.

I can be holding 4 min/1k pace. I can look at my lap pace at the end of my 1k and i can be 5-10 seconds above or below 4min/1k pace but the entire duration of the interval the instant pace is just nowhere near this. It will show 3.30min/1k or maybe next update it might show 4.45min /1k pace.

I have had a little hunt across the forum but couldn't find anything else about this, although I suspect I've just not located it.

Has anyone else experienced this?