Overheating

My '95 Z28 with the LT1 350 runs fine for awhile but then after it gets up to what would be normal temperature the guage climbs fairly quickly and then pegs all the way to high and then the coolant boils out of the overflow...this car has recently had new waterpump, thermostat, radiator, hoses...cooling fans are operating...no water in the oil...no steam going out the exhaust...tried running without thermostat and took longer to do it but did same thing...tried some fancy stop leak additive but no good...when I changed one of the heater hoses I noticed there was a small plastic valve that appears to be a one way check valve going to the heater...this valve was broken so I eliminated it and just put a splice connectoe in it's place...don't know what the purpose of the valve is but don't think it would have anything to do with what is going on because it did it before I removed it..anyone have any ideas?? I am sort of suspecting a crack that opens up when it gets warm enough allowing compression to go into cooling system...or some people think it could be a head gasket...others even told me it could be an intake gasket or even a kinked or plugged fuel line??? I am in the process of tearing it down right now but haven't got the intake even off yet...these cars are fun but a pain to work on for sure...any help would be appreciated

My '95 Z28 with the LT1 350 runs fine for awhile but then after it gets up to what would be normal temperature the guage climbs fairly quickly and then pegs all the way to high and then the coolant boils out of the overflow...this car has recently had new waterpump, thermostat, radiator, hoses...cooling fans are operating...no water in the oil...no steam going out the exhaust...tried running without thermostat and took longer to do it but did same thing...tried some fancy stop leak additive but no good...when I changed one of the heater hoses I noticed there was a small plastic valve that appears to be a one way check valve going to the heater...this valve was broken so I eliminated it and just put a splice connectoe in it's place...don't know what the purpose of the valve is but don't think it would have anything to do with what is going on because it did it before I removed it..anyone have any ideas?? I am sort of suspecting a crack that opens up when it gets warm enough allowing compression to go into cooling system...or some people think it could be a head gasket...others even told me it could be an intake gasket or even a kinked or plugged fuel line??? I am in the process of tearing it down right now but haven't got the intake even off yet...these cars are fun but a pain to work on for sure...any help would be appreciated

That's weird, because if you fans working, your car shouldn't technically be overheating. Might want to check your intake...just going out on a limb. Check your coolant as well, maybe to much water????? I dont really know though.
Good luck