Dry hopping in primary + yeast harvesting = bad??

[sbaclimber]

My current brew-improvement-mission is to reduce the amount of unwanted/unpredictable esters which keep occurring in by brews.
I was recently able to effectively eliminate too high of a fermentation temperature as a cause, by buying a refrigerator. My last brew was kept at a constant 18°C for the first week of fermentation and only raised to 20°C for the last few days.
Unfortunately, I still ended up with quite a bit more esters than I was expecting....
Remaining possible causes are under-pitching and stressed yeast.
Considering I only use fresh yeast every 3rd brew (the other 2 are pitched using yeast harvested from the previous brew), and the esters have been occurring the strongest in the brews fermented using harvested yeast, I am fairly certain that I am at least under-pitching. The Starter Calc. here tends to back up my suspicion, though it is hard to be 100% sure, because I don't really know how much "good" yeast I am harvesting. Either way, my next step is to eliminate the possibility of under-pitching by building a stir-plate and boosting my starter by a lot.
What I am not so sure about is.....stressed yeast. I always pitch horizontal (same OG) or up, never down. So that shouldn't be an issue, but I do dry hop in my primary (thanks to Larry for the time and effort saving tip! ). I recently read something *somewhere* that seemed to suggest that yeast shouldn't be harvested from anything containing non-cooked hops. Is that a real risk? ...or could it be, that I simply misunderstood and shouldn't worry?

EDIT: PS, some general background info...
I always brew IPAs using WLP007 and primarily German Pale Ale and CaraFoam malts.
Most of my brews are ~14-15°P OG and I normally pitch yeast into 1.5l. 9°P wort for 24hrs to make a starter.
My last brew was a 19°P OG IIPA (slightly more % crystal malt than normal) with a 2.5l. starter, using 2nd generation harvested yeast.

 

[Ozarks Mountain Brewery]

using a bag for your hops would be better, too much hops seem to cause yest to not perform well, bad flavors can also be caused by a small infection or unsanitary washing of the yeast, drastic temperature changes or the hop trub mixed with the yeast, using aged yeast too.

also never save yeast from a beer that doesn't taste right

try changing your system up and pitching a couple dry yeast once or twice just to see if its your washing and using a bag for dry hoping

 

[sbaclimber]

I always use a bag for dry hopping, and filter my wort after boiling, so I don't have any trub.
My washing process isn't exactly optimal, so that might be something...

 

[Ozarks Mountain Brewery]

one thing to note that hops mildew very fast, if not completely submerged under the beer can have a drastic effect on the beer and yeast

 

[sbaclimber]

one thing to note that hops mildew very fast, if not completely submerged under the beer can have a drastic effect on the beer and yeast

That is good to know. I weight the bag, so it is submerged, but never really thought too much about the "why" I do it (just seemed like a good idea)...