Have had an 87 GN, 63K, for the past year. Basically stock with a few goodies. Love the car! But having a sputtering engine. More like a "bucking bronco". Even with engine warmed up, it will buck. Seems to hover at about 35mph when it does this. Will improve as car warms up but has done it at optimum temp. Anything I should start looking at? Injectors? O2 sensor? Any help I'd sure appreciate!