96 olds aurora

My olds aurora keeps overheating. In the coolant resorvoir I found a rubbery black substance. What is it? Ive replaced the thermostat, water pump and radiator. What next?

Have a block check done to see if exhaust gases are getting into the coolant system.

The black substance in the reservior could be the result of the cooling system hoses breaking down internally. If this substance is found in the reservior, one could assume it’s also accumulated in the radiator which may be restricting the coolant flow through the radiator, resulting in the overheating.

You might try replacing the cooling system hoses and then have the cooling system flushed to see if it eliminates the overheating problem.