i changed my housing to and ever since i changed i had been experiencing call drops and dropping of signal strength. then i did a deep analysis and guess what the antenna was attached to the old housing and i had been using the phone without the antenna.
then i removed the plastic part from the old housing place it in the new housing.
http://www.xperiax10.net/2010/05/10/xperia-x10-disassembly-guide/
check "Step 4: Remove lower support" 4 the picture of the antenna.
hope this helped.