You pretty much got the exact idea for re-learning FFR. FC songs that you can comprehend without mashing, AAA songs that start to become susceptible to being so, try different speed modifications, rinse and repeat.
You'll eventually hit a point where there will be very few songs to practice on, mindblocks will start settling in, and improvements will slow down. A majority of people look to rate mods, FFRmania, or Stepmania as different outlets & sources to improving past what the regular FFR engine can offer.
FFRMania:
http://www.flashflashrevolution.com/...d.php?t=132902
Stepmania Packs:
http://www.flashflashrevolution.com/...d.php?t=133223
FFR Rates can be activated in Options when using Flash Player 10 or above on the R^3 edition of FFR.
If lag is the issue, consider switching over to using a Standalone version of the game if you haven't already.
Tutorial on how to set that up is located here:
http://www.flashflashrevolution.com/...d.php?t=120256
Beyond that, the majority of fixable solutions include patience, dedication, and hope. If you play once a day for maybe an hour or two per day, you're almost 100% guaranteed to skillboost by the end of 2 weeks or so, at least imo.