Fireman sam, i think i know the taste you are refering to. I couldn't actually describe it but it has been in every single kit brew that i have ever done. I recently put down an all extract brew and that taste wasn't there. So maybe give an all extract brew a go, its not that much more expensive and the beer tastes way better imo.
I agree. My first few kit beers all had a funky after taste but as I went on and learnt a few different techniques over the years I have managed to hide most of it.