tach bouncing???

ok i was wondering if someone would be able to help me out with my car issue... so here i go... my 69 383 under full acceleration, the tach bounces/ or twitches, i only notice it once in a while.. the second issue im having, is that while driving down the highway at a constant speed it feels like the car is pulling, then coasting for a brief fraction of a second and happens over and over again but i dont notice it under acceleration whether its hard or normal just slightly pressing down on the accelerator..and i have no idea what it could be, im open to suggestions if any one has an idea... thank allot guys!