Occasional hard shifting?

About half the tike when I drive my car(2009 challenger rt 6 speed) it shifts smooth as butter and the other half the shifts are notchy and the car jerks a bit. If it's not doing it all the time it wouldn't be synchros would it? Car never grinds gear. Have recently done flyid change to RP synchromax. I'm stumped...this is only in regular driving scenarios, when I'm getting on it the gear changes are smooth.