I'm kinda noticing a similar effect with methylphenidate (Concerta and Ritalin) actually. The longer the period I take it over the speedier things get, but I also get more scatterbrained. It's not a like a reduction in intelligence, it's more like things are less organized. I take mine for ADD too, but at this point it's looking like it's mainly a work aid. As long as I'm not taking insane doses, methylphenidate seems to build tolerance very slowly over time, which can be reset within a week if you're prudent. Note that I'm only taking 1-3x the prescribed dose by my doc at a time, and only taking it orally.
If you're taking the amphs by oral ingestion only, tolerance and some withdrawals can occur but I don't think it's as bad you see here >>4048
. My guess is that methylphenidate would be easier on your brain and is less addicting, but I don't know what swittching from addy to ritalin is like.