Extract beers don't last?

I was wondering if anyone else has experienced this. I have an amber ale a brewed back in the beginning of September. It was tasting fine the past few weeks after bottle conditioning but there is a chemical like taste starting to develop in the bottles. I have had this same flavor happen with all of my extract brews. If I brew all grain that flavor does not show up. I was wondering if this is a common thing or if there is something I can do to "prolong" the shelf life of my beer.

Quick primer on my extract process:
-I use RO water that I add brew salts to that are supplied by my LHBS. hey have them pre mixed and packaged.
-I do full boils and do not top off
-The most recent recipe had three additions of cascade hops. A 1.5 oz 60 minute addition, a 1 oz 15 minute addition, and a .5 oz flame out addition.
-My fermentation is temp controlled and I made it a point to ferment this beer at 67F
-I sanitize everything that comes into contact with my beer post boil using star san at proper dilutions
-All of my extract batches use steeping grains, these were steeping at 155-160 then drained.

I think that should cover the major parts of my process just in case anyone thought it could be bad practices.

Any help is appreciated guys!

Buddy had a same problem found out it was to high of temps for storing bottled beer. Just a guess ! Me i just drink it sooner. well as soon as its bottled.

I have some from last april or may and some from june that taste awesome

If I could drink more I would, but I pull a lot of LONG days thanks to full time job and full time school. I don't feel like drinking a beer when I have been going non stop for 17 hours straight. All I want is a warm place to close my eyes for 8 hours.

I would think that you are allowing either too much light into the beer or storing it at too high of a temperature. Both produce a reaction with your beer causing off flavour to occur. Or you could have a problem with the storage life of your LME or DME. DME last longer. And it may not be your fault but where you got it from not being fresh enough.