If it tastes like metho it could be fusel alcohols. Higher temperatures are usually the cause as well as insufficient yeast or poor yeast health or lack of oxygen to the yeast early during growth.
This page might help you identify what you're tasting - unstable temps can also lead to the production of esters, some of which taste like fruits and some of which taste like nail polish remover (acetone).
http://www.homebrewzone.com/off-flavors.htm
Get a fresh tin.
Get some new yeast that is within useby and has been kept in the fridge. Try safale US05 which comes in 11.5 gram packs and ferments clean and neutral.
When you mix the goop and water use a clean sanitised whisk to aerate everything
Ferment at 18 degrees, constantly until finished then leave for another week.
Make sure you leave the bottles to condition for a good few weeks (try 4 weeks)