My local Autosupplies has stopped supplying aftermarket coil packs, the quality is terrible and they fail regularly.
Genuine Ford ones (cost me £107 over the counter) seem to work much better and have so far cured the poor running problem on a 2004 1.6 Zetec Focus. I hope!
I work on classic cars and the quality of parts especially from India or Asia is dreadful; rotor arms last about a week.
Buy the best replacement parts you can and the chances of fixing the problem are much higher.
UlsterTriumphMan
November 2008