Sour flavor the result of yeast or sanitation problems?

[YukonCornelius]

Long-time listener, first time caller. Been brewing for about six years. Almost exclusively extract and bottle ferment. Having this intermittent problem with high gravity brews, hard to describe the off flavor I'm getting but I shared one good and one bad batch with a local brewpub brewmaster and he felt it wasn't infected or sanitation related. Hard to know for sure. A little sour, a little bitter, not overly sweet, not farmhouse saison banana estery/peppery, best described as a little sour, bitter and artificial/plastic.

Theoretical recipe goes something like this:

2 parts HME
1 part Honey
1 part fruit

If I'm using liquid yeast I've been using a variety of Wyeast high grav packs, belgian trappist etc.
If I'm using dry packs I've been using Safale T58 or BE256, their recs for high grav tolerant yeasts.

Things I've changed have been

1) leaving in primary for two weeks and then bottle ferment.
2) Racking off into secondary after high krausen maybe 1 week in primary
3) Changed yeasts
4) bottle conditioned for up to six months (I had one that sat for two years that I tried last week, 50% of it was great and the rest was off.
5) Changed sanitation chemicals (mrbeer included, generic oxyclean, PBW and star san)
6) Used distilled water and added appropriate electrolytes

Now, if I was being impartial and read all of the above, the answer would be that the sour/bitter/artificial flavor plus the intermittent nature of the problem suggest infection. I'm not too big for my britches to admit this, but what I can't figure out is how it's varying from bottle to bottle, when I'm an absolute nazi about sterilizing the bottles.

Is this a sanitation problem or a yeast problem?

Thanks for listening.

Edit to add: It would be very hard for me to bring fermentation temps down below 68F, don't have spare fridge space.

Edit #2: Over-innoculation?

 

[thehaze]

Sour could be low pH during mash and most important in the final beer. Belgian yeast will produce peppery/clovey phenols and some mild tartness and a dry-ish mouthfeel, depending on final gravity and mash temp./sugar used. High IBUs in belgian beer would feel more bitter than say a overly hopped ale, so maybe adjut the IBUs a tad next time. Get a pH meter and get a hold of RO water or bottled water, with a balanced ion composition.