[Accelo]

It almost has to be water. Is the coolant level dropping? You can also have the coolant checked for exhaust gases or can purchase the kit and do it your self.
A leak down tester may help.

A stuck injector would likely hydraulic lock the motor. Many of the scan tools will let you disable injectors one at a time. Then you watch for an rpm drop.

A compression test may help. I have two style compression testers. One has a schrader valve that only allow air in. The gauge stays at peak cylinder pressure till you release schrader . The other dumps everything from the hose , gauge and cylinder. So when you quit cranking the pressure starts falling immediately and continues till the pressure in the cylinder is zero or you push the schrader and dump the pressure. Using the second type gauge you can watch as the cylinder pressure drops you can kind of calculate the condition of the valves and rings (and any leaks in the head gasket).
I just did a compression test on an LS1. Cylinder pressures went to a nominal 200psi. Then, when I quick cranking, I timed the seconds it took to drop from 200psi to 100psi. A good cylinder would take 60 seconds to drop the 100psi.
This way I got a compression test and a cylinder pressure drop test all in one.
Do this twice for each cylinder. If the pressure goes up on the second try remove the gauge and crank the motor. I have shot water half way across the shop using this method.
Best of luck