My pace ranges between 7:30 to 10:00, fluctuates every second.

the Gps is so bad. had the same problem after changing from Forerunner 735 xt to F5plus. Now I ordered a stryd and I think this problem is solved

the Gps is so bad. had the same problem after changing from Forerunner 735 xt to F5plus. Now I ordered a stryd and I think this problem is solved

Yeah, as of 8.01beta (5X Plus) GPS sucks (painting zigzags and projecting me 20-30 meters to the side), OHR sucks (out of 3 runs it does a perfect run one time and then two weird runs, with just showing random numbers which may easily be 10-15% off and imitating even running wherever I'm actually standing still or accelerating as fast as possible. So there is no single chance it will correctly respond to interval training of any intensity). I didn't think this quality is possible, especially at this price, in 2019, I also was sure those pace fluctuations you mention have been removed from modern watches long ago. But that's not the case. And unluckily, other options on market, Garmin or not, seems to be even worse.

Not a fix but a workaround.

You can download the "Rolling Average Pace" data field from the IQ store. Works pretty well for me.

Another way is to use the the average lap pace.

You could also try using a footpod, I don't have one but from what I've read the pace derived from them is pretty accurate.

The average pace is always out at the start of a run, it takes the watch at least 5 minutes to sort itself out. I am going back to my 735 and returning my Fenix 5 the 735 was more reliable