I had the same issue when my VCT solenoid was bad, my timing was like, half a degree off TDC, which isnt bad because the VCT should be able to adjust for such a minor inperfection, but when the motor would get warmed up, especially on hotter days, it would buck completely randomly, it helped if I drove it very slowly, but overall it would buck whenever it wanted too. Basically, you need to replace the VCT solenoid, or find a way to eliminate it, but it sounds more like that even if you successfully eliminate the VCT it will still buck and hesitate, if thats the case, then your timing is off.