Were you having knock issues with stock heat range plugs (indicated with a scan)? Or just threw a colder plug on for the heck of it? The Army's got a saying: "If it ain't broke, fix it 'till it is." You may have only needed fresh stock plugs to fix your problem, not colder plugs.

Though, at 98k, could be a clogged fuel filter (should change this anyway - it's $15), if your MAF sensor and throttle body are clean.