Just wanted to share this.
Yesterday I did a endurance ride that lasted about 2 1/5 hours where I followed my trainer i ERG mode until 90 minutes had passed. The difference in Trainingpeaks between the first 90 minutes and last 60 mins was weird when I looked at the analysis. Although I had higher average watt the first 90 minutes (watt 175/hr avg 127) and lower avg watt the last 60 minutes 173 watts I had a staggering 135 avg HR. What I observe is that my power readings are fluctuating a lot when out of ERG mode and I makes med burn more matches.
I take with me that I might need to be more focused when out of ERG mode.