Well how is the engine running? Is it rough wont stay running or run smooth like there is nothing wrong with it? You can also do a compression test of the cylinders to see if there is one or two that is a lot lower than the rest.
Just thought of this does the radiator stay full??
Could be a leaking intake manifold.
Try pressure checking the cooling system. Autozone rents pressure testers. If that doesn't narrow it down then do a compression check. You might also pull the plugs and see if there are any that are wet. That could narrow it down to one cylinder or one bank.
I bought the car here off of racing junk, drain the oil two quarts of clear water came out first followed by thick sluggie light brown liquid, radiator was not empty 3/4 full