heat plug problem ?

please does any one know why my 460 gd heat plugs keep failing ?about three months ago starting became lumpy to say the least so i pulled out the heater plugs and found three of them not working ,these i swaped for some good ones out of another engine these worked for another 3or 4 weeks untill it was starting on two cylinders again .so i decided to buy 5 brand new bosh plugs from our local factors , all was well untill yesterday ,it sounds as if one is of again ,i think i have read somwhere that carbon deposits can shorten there life is this true ?when ive changed plugs before they last at least a year .
TIP its much easier to remove the injector pipes before to acces plugs