Off flavors

I brew many ales and lagers. 
I am struggling with off-flavors of plasticy or chemically flavors especially in my Lagers. My ales will sometimes exhibits this problem to lesser extent but my lagers consistently do to varying extent (I recently made a Bohemian Pils that was not too bad and was drinkable but then I did an Oktoberfest that was horrible).
I exclusively brew all-grain and keg. 
I use a non-bleach "no-rinse" sanitizer and still rinse to be sure. 
I have a lager fridge to do it proper.
I have heard that oversparging can do it so I recently tried a no-sparge mash but it is too early to tell if it was successful.
If oversparging is the problem I have read extensively about it but have found no consensus as to what exactly is oversparging.  Is there a volume or temperature threshhold?
Any help or input would be great.

JAbrews,
            You might try using different types of sanitizers. Rotate brands. I think if you use the same sanitizer continuously bacteria may build up a resistance to it and you will grow a "house infection" that will be resistant to your standard sanitizer. I had a few batches of beer that were bad and I didn't figure it out for a while. I was rinsing the kegs with water "after" the (no rinse) sanitizer "Just to be sure?? My water is from a well! It was loaded with bacteria! I have know idea why I suddenly did that?  Needles to say I never re-rinse my kegs and I have been free of any off flavors. I don't have to sparge because I Brew In A Bag. I know things will work out for you, good luck!

JABrews said:

I use a non-bleach "no-rinse" sanitizer and still rinse to be sure. 

Click to expand...

No-rinse is no-rinse.......don't worry about residual effects you can taste or smell.  Your tap water IS worse.

Soak everything you can in OxyClean or PBW to ensure it is clean, and then soak in sanitizer.  If you have doubts, replace that tubing, or whatever. 

Take all your ball valves completely apart and clean them well, then sanitize.  Picture is of a ball valve after ONE use, and after it had been cleaned.  The gunk does get behind the ball. 

+1 on the taking stuff apart and soaking. I was shocked at what I found inside the guts to the quick-release posts on my kegs. Sanitation is a cornerstone. RDWHAHB when it comes to Star-san or any no-rinse.  It sounds like you are a serious and experienced home brewer so it may just be a mystery. Do something totally different each time you brew until the problem disappears (within normal brewing practices).
Also, revisit your mash temps/time, methods, water, etc. Read the gurus again (Palmer, Papazian, etc.) 
You may be doing everything correctly, or you may discover something that could be done different.

Do you buy grain fresh or your old stash? Does the supplier have fresh stuff? Are there kids or people who may be messing with your stuff?

I brew weekly and I order from Austinhomebrew.com which has a lot of turnover in their product so my grains are pretty fresh.  At most, my grains hang out in the closet for a month sealed in the bags they are shipped in.  Kids and dogs cannot get into it either.  Thanks for all the input.