Transmission shifting issue

I bought a clean magnum, sometimes it starts off in second gear and sometimes it stays in second until it almost to red lines then I let off the gas and go normally then it will shift into third. Ninety five percent of the time he wont shift into fourth gear. Sometimes, rarely at 80-90 km/h it feels like a mini overdrive that lower the rpm's from 2300 to 2000 or 2100, but it is not in fourth gear.