Additionally, the timeout of 3.5G -> 3G drop is controlled by network provider's side. For example, in Poland, Aero2 BDI provider tries to drop to 3G every two seconds, while T-Mobile does so every nine.

You can force being in 3.5G constantly, by experimenting with ping and ping packet size. In the above example, T-Mobile require smallest possible ping every 8 seconds, while for Aero2 BDI, only 1KB-sized ping every 2 seconds guarantee staying on 3.5G.

The down side for mobile device is that, keeping 3.5G, you would get very (veeery) poor battery life. It taxes device with 300-400 mA current draw during usage. forcing 3.5G maxes sense on modems connected to desktops or, in some cases, laptops, though, as it ensures low pings (3G pings to common server - like, google - can get as high as 900 ms, 3.5G ones depends on provider and situation, but can be as low as 40 ms).

In your problem's example, everything seems to point a finger on provider being responsible for your problems. That is, if signal strength is decent.
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Arien Stokowiec