The white stuff could just be from the cleaning agent. Did you flush the cleaning agent out after REALLY REALLY GOOD? If you didnt this will happen. Disconnect your upper radiator hose from the block and put a garden hose to it. Then uncock your radiator and let the water flow until it is crystal CLEAR. Then use some good coolant, most people would say use Honda and thats what I use as well, but just use anything that is name brand at least.
Do you overheat?
If you do and you think its your headgasket you can buy a Combustion leak detector or take it to a mechanic and ask to have it done. The kits are online for like $50. It looks like this
http://www.arrowheadradiator.com/hea..._leak_test.htm
To me it sounds like if your radiator cap is leaking you need to replace it. Does your upper radiator hose feel harder once your car has reached operating temperature?