Pretty much have the exact same problem! I played very well all through summer, now that September is here.. Wow. Not so well. Didn't break 80 for the first time in 6 months yesterday. What a disappointment. 

I'm with you and it's been like that the last few years..  It's like I forgot how to play golf...  

I haven't found a cure yet....   I just keep plugging away and having fun...  Winter is almost here so I try not to let it bother me too much...  Just enjoying whatever rounds I can get in....

Good topic though.. 

I think it's pretty common.  Most of us are done with our "competitive" golf for the season and we know the end of the golf season is approaching (at least those of us that have a winter).  I feel like I hit a lull in my mental approach to the game every year about this time...even though I still want to play the best that I can (especially since I'm still posting scores for handicap purposes).  The rounds that I'm playing aren't competitive, so it's harder to grind it out.  This lull usually lasts a couple weeks, then I get back on track until the true end of the season.

I can say i go through the same thing this year was a little different though, playing a awesome tournament in Vegas I had a friend vidoe my swing to send to my son and noticed  some things didnt look right my friend said yes that has gotten worse over the past month, So I think we all create some bad habits over the time we are playing the most and if I wouldnt have seen the video I would have played the rest of the year with them and wouldnt have changed them until the begining of next year. After I seen the video and made the changes I am hitting the ball better than I have in 3 months. So hopefully the end of this year the handicap wont go up 3-4 shots last the last few years have.

Just focus on the basics. When I feel my swing is off I like to go on the range and hit half wedges to a flag. This helps the tempo and will slowly translate into your full swing. Thats my goto when I am in a slump or not trusting my swing.