So if the foam, not air bubbles, isn't dying down then maybe the speed is too high. http://www.alphacool.com/product_info.php/info/p808_Alphacool-VPP655---single-edition.html
That link is to your pump, I believe it is a manual pump i.e. you have to adjust the speed on the back of the pump there should be a little red screw thingy that you have to rotate to speed-up and slow-down the pump. If not then it is a PWM pump, there should be two connectors coming out of the pump - a 4 pin molex cable and a smaller fan cable, this should be plugged into your motherboard. Install your motherboard software and you can adjust the speed manually via the software, alternatively enter your BIOS [DEL, ESC etc on startup] and change the fan speed settings there.
If the problem persists then it may be due to your coolant or the mixture. Did you do a parts clean beforehand, i.e wash everything out? Is the coolant mixed properly, i.e 1 to X or whatever? If unsure and you have some spare coolant then drain and refill.
If the problem persists I would recommend using distilled water with and additive to see if the problem is replicated.
Hope this helps!