Garmin HRM TRI heart rate readings incorrect

So my old HRM dual transmitter literally fell apart a few weeks ago, so I upgraded to the HRM Tri. 

On every ride and run I've done with it, at varying points it decides to go off on its own and shows a dropping heart rate, before realising it's wrong and jumping back up to the correct rate. 

I tried the whole pull the battery out for 30 seconds deal which didn't fix it - so I took it back to the bike shop and had it exchanged for a new one.... which did the exact same thing. 

No matter the device its connected to (Zwift, Edge 520, Forerunner 735, a combination of the three) it seems to do the same thing. I've attached a screenshot of my indoor ride on Zwift this evening. You can see at least 4 points where it goes haywire. 

Garmin's response was to follow these steps on this link - My Heart Rate Monitor is Reporting Erratic Data 

Despite this, none of these steps seems to fix the issue. 

Has anyone experienced this before? I'm on my second strap and they both do the same thing, so I'm wondering if there's a firmware update or something that I'm missing that might perhaps fix this issue.