I would like to ask a couple questions that might be of some use.
First, does the same issue occur when using the phone with the flip closed? That is, can you use the jog dial and keys on the flip properly?
If the phone works fine with the keypad and jog dial, then you should be able to try to recalibrate the touchscreen.
To re-calibrate the screen:
1. Select Applications tab
2. Select Control Panel
3. Select Device
4. Select Display or Screen
5. Follow the instructions on the screen.
6. Select Yes or No
Second, if you turn the phone off and on again, with the flip open, does anything load by itself, or does the touchscreen react as if something is being pressed?
This may indicate that there is a problem with the hardware.
It is possible to purchase a new touchscreen for under $100 from a few parts retailers such as http://cnn.cn
It may also simply be a glitch or bug in the software or firmware OS. In this case, you can try a master reset or updating the firmware using Sony Ericssons update service (not compatible with Vista for the P910), or you could use a third party firmware utility such as from DaVinci etc.
If you choose any of the master reset, firmware or update options, you will lose ALL user data on the phone, so you should back it up before you proceed.