1999 Chevy Blazer Misfiring.

Hello. I seem to be having a "misfiring" problem that no one can help me out with. My car used to only slightly misfire when it was cold or wet, but now it does whatever the weather is. The problem is that it doesnt misfire when idling, only when driving. If I floor it, the "misfire" goes away. My check engine light has been on, and as far as the scan code comes back as "multiple cylinder misfire". It used to only lightly misfire, but as of late it has gotten bad. Also, the plugs and wires were replaces a year and a half ago. Please help!