connections started with HSUPA modems are unstable

I've tried four 3G modems with 10.10 ubuntu distribution. Two of them are HSDPA while two of them are HSUPA. Following are the models.

modem 1: Huawei E156G, HSDPA
modem 2: Huawei E143, HSDPA

modem 3: Huawei E220 HSUPA
modem 4: Option Icon 431 HSUPA

 When i connect through HSDPA modems, everything is fine. The connection was stable and i always got an upload speed around .1Mbps. (checked through speedtest.net). However when i connect through a HSUPA modem, the connection showed following problems.
1. upload speed is .05Mbps, which is half what i get with HSDPA modem.
2. connection is not stable. it gets disconnected often. just after a disconnection, if i retry it works.
2. speed continue to reduce during the lifetime of the connection. Just after connection, it gives a good speed, but gradually reduces to a point where i can no longer do any useful work. If i try to load a page in the browser at such a time, i see the message "sending request" in the status bar of the browser for long time. It seems like the modem has problems sending data to the signal tower. The average signal strenghth i get is 32%. The modem heats up a lot when connection is live.

All these made me think that this could be due to insufficient power to the modem. I think HSUPA need more power to keep the higher upload speed. In fact, the modem 3 (huawei E220) has two cables to plug in to USB to draw more power.

here are few other detail that might be useful.
i mostly used the wvdial with following config.

#Modem=/dev/ttyUSB0
Modem=/dev/ttyHS3
Baud= 460800
Init = ATZ
Init2 = ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
ISDN = 0
Modem Type = USB Modem
Phone = *99#
Username = s
Password = s
stupid mode = 1
New PPPD = yes

i have used usbmode_switch package for plug and play support.