As mentioned above by others, the fault may not be with the spark plugs but another component that is the root of your problem.
However, if you do replace your spark plugs, its worth noting one thing. Its best to be very careful when changing to another manufacturer/upgrading your spark plugs. Modern cars equipped with all these fancy management sensors etc use spark plugs as a communication port into the combustion process and can act as a sensor as a part of a highly sophisticated system. So in some cases it is wise to only use the vehicle manufacturer's specified spark plugs (my cousins BMW V10 M5 is a prime example of this as it will only run on the NGK LKR8AP plugs).
My BMW 328i for example, runs fine on standard NGK and Denso plugs, but feels less responsive on Bosch Super Yytrium.
In the trade, we mainly use NGK and Denso spark plugs due to the high quality of manufacture but we have never been let down by any Denso spark plugs, only every impressed! (Always ran my Supra TT on Denso, I am putting these in my T5 next service!) As far as Bosch plugs go, we are happy to use them but only the standard specification ones nothing uprated. (T5's have a very high quality platinum plugs fitted as standard anyway I believe).
Also, just for your information you may want to take a look at the following sites.
http://www.globaldenso.com/PLUG/ (Denso spark plugs)
http://www.ngkntk.co.uk/ (NGK)
http://www.boschautoparts.co.uk/ (Bosch)