Bitter Oregano

I've been growing your Greek oregano for several years and every two years or so the leaves get bitter very early in the season. So, I have to replant the oregano frequently. Is this normal or can I do something to prevent this? The herb garden is east facing with sun until early afternoon, soil is average and water is by drip system which I turn on as needed. For organic matter I add some peat moss each spring. I'm gardening in zone 3 in interior British Columbia at about 4000 feet elevation.

In our own fields, we do not see the degradation of oil quality you describe. There is always a natural variation in oil content and quality depending on the season, and we wonder if you would get better flavour or odour later in the season. Have you tried keeping the bitter plants until the hot sunny summer and checked them then to see if their taste was as it should be? Different oils would be produced under different conditions in the same plants.
We have no other explanation for the phenomenon you reported. At least one other grower has reported similar results so we are aware of the problem. It is difficult to investigate further without actual specimens that have deteriorated as you have described.