That sounds like your problem is torque converter drain back. Could be from leak in converter (weld, perhaps) or a bad seal around the converter. If the converter can hold a vacuum, it will keep its fluid inside of it, for quite some time. If they will not hold a vacuum, they will allow a good portion of their fluid to drain out of the converter, back into the pan, often also showing dipstick and shifter/selector shaft seal leaks, as the fluid level is high in pan after a short period of time. A Torqueflite 727 doesn't pump trans fluid in park, which is why you need to check your fluid level in neutral. So, once placed into a gear that DOES pump fluid, the torque converter must fill itself back up before it can build enough line pressure to move your vehicle. When you feel the surge is most likely at the moment that your torque converter is again full of ATF and builds line pressure.