This is a normal function of the HSPA networks. When there is not the need for a high data rate the device switches back down to 3G. When there is demand then it switches up again, but there is a delay.

You need either a program or a script to keep pinging something to keep it in HSPA mode. Other uses on the HSPA network won't like you though as you will be using up capacity on the network that you aren't using.