1986 chevy transmission problem?

i just bot a 86 chevy lifted its a nice truck but only one prob with it the transmission is acted funny. when i go down the road it shifts find in first and 2nd gear but when its time to shift in to 3d and over drive its like the just stops working and the egin revs way up what could be causing this? i do not have the transmission cable hooked up to the carb but someone told me that that's just for when you step on it it will shift down. ummm so yeah what could be causing the trany to go fine in 1st 2nd but then when time to shift to 3rd it wont shift it just stayes rite in 2nd and when i get to a serten speed its like the trany completely disengages ?????? thnx for your help