What guts did you replace, and with what? I'm assuming you mean you replaced both the fill valve and the flush valve.
You could just be unlucky enough to have a defective replacement flush valve or the chain is too tight.
The common causes of this are (in sort of descending order), as most have said above:
(1) Needs new flapper;
(2) Needs to clean flush valve seat with brillo;
(3) Chain too tight (keeping flapper slightly off seat);
(4) Chain too loose (slipping under flapper edge);
(5) Needs flush valve seat repair kit;
(6) Refill hose stuck down into overflow riser;
(7) Crack in overflow riser (usually a vertical crack that starts under where the clip for the refill hose is attached);
(8) Leaks around tank-to-bowl bolts or around flush valve connection to tank.
Before you replaced the parts, I was going to say that if it falls to 1/2 tank and stops there with the water turned off at the wall, and you used the flush valve repair kit, it was going to be a crack in the overflow riser or something about the flush valve seat or the chain. You want basically one link of slack in the chain to droop on the top of the flapper with the trip arm at rest.
If it wasn't those things, some more esoteric possibilities are: (1) there is a leak somewhere around the flapper or flush valve seat that is miniscule, and when the tank level drops (and thus the head pressure at the valve drops), it is able to seal; or (2) you have a toilet (like one of mine) where when you put the lid back on the tank, it slightly-depresses the trip arm, tightening the chain and standing the flapper off the valve slightly, so a small amount of water flows. That last one is a huge longshot, but maybe worth mentioning.
The bottom line is just to check all the possibilities. You have a china vessel that has just four holes in it and gets filled with water. One hole isn't relevant if you have a modern anti-siphon fill valve, two holes (the tank to bowl bolts) would leak onto the floor in most cases, and any cracks in the vessel would leak onto the floor. So the only place you really have to focus is the hole for the flush valve and what you have stuck in that hole to block the water. If you have given the big plastic nut under the tank a hand-tight turn and then another quarter-turn, that should be enough, and generally you can see if there is a leak there. Sanding the bottom of the tank lightly around the big hole and around the bolt holes to give a smooth surface sometimes helps, but most of those leaks are obvious, and your leak isn't obvious. So that leaves you with the flush valve/overflow riser itself, or the flapper and its ability to stop water. You might be able to test the flush valve by disconnecting the flapper from its chain and pressing it securely into place, then seeing whether there is a leak. Sometimes, the flapper doesn't drop squarely on top of the flush valve, because either it isn't attached right at the ear/hooks, or because the chain pulls it to one side or the other.
Me personally, I am guessing that there was a problem with your prior flush valve (like a vertical crack that you could find by pinching it at the top), and you replaced it but induced some new issue with the new flush valve, perhaps something related to the flapper or chain length or how the flapper is attached or drops on the flush valve seat.
Shoot us a couple of photos of your pristine new setup, and let us see if we can see anything obvious.
We will work it through until you get it fixed.