You were correct. It is from babying it during break-in. During break in it is important to have high MEAN(average) EFFECTIVE PRESSURES and not necessarily high peaks.
In other words, engine power settings at greater than 75% is generally considered the goal. So lets assume that is 225HP on a 300 HP engine. You want the high MEP rather than a high peak for two reasons, one has a greater sweep of piston ring travel with a highish pressure and you want the one that has the lower CHT.
This is where many LAME's and all but a few engine builders come unhinged, because the best way to achieve the better break in is using a LOP power setting. Of course some engines straight out of the box will not have good F/A ratio's and thus will not run smoothly LOP. If you can get that sorted at the beginning that is the best way. If you can fit a set of GAMIjectors and go for it, even the first cut is usually good enough, and you can tweak them during the break in period. Which by the way is really 5-6 hours and certainly by 10.
If you can't get a smooth engine (good F/A's) then do the break in at say 75-80%, which generally equates to 5000' or 24" and 2600 RPM in most engines and FULL RICH.
Vary the RPM occasionally and don't be afraid of it. Not a good time to baby it.
One method has cooler cylinders, better mean effective pressures over a greater sweep and is by far cleaner on the combustion chamber etc. One is lighter on your wallet too!