Spark Plug Problem in 95?

i have a 1995 tacoma and there is a problem with the spark plugs,they seem to be misfiring. i have replaced them once myself and my truck ran AWESOME for about 6 months and then went back to having problems again. i took it to the dealership hoping they would have some insight, but all they did was replace the plugs and wires and now 6 months later it has been slowly getting worse and worse. any advice? PLEASE!!