You may not have a perfect seal after the gasket change and be losing compression (which might be why it's worse as it heats up and expands. Try a product called 'KSeal' it costs around �10 and you add it to the coolant, there's no need for any preperation just add it cold and run the engine as normal. This will find and seal any leaks that exist or develop in the future, even a cracked cylinder head! I used it on my car when the head gasket had blown (white smoke pouring from the exhaust and a total lack of power) and a year on the car is running beautifully (saved me a fortune!). KSeal can be quite hard to find in the shops (Halfords etc don't stock it) I found it in a local specialist car shop, but it can be found online. It comes in a small blue bottle with orange label. Hope this helps, for �10 it's worth a try. I will be adding it to every used car I buy in future.