I had a similar issue when building a W520, before I added the palmrest, the trackpoint would work intermittently. When I got a palmreat and attached the touchpad, the synaptics drivers started working, and the trackpoint was working good.
Perhaps the connection for the touchpad is coming loose, and causing a similar issue? Maybe try reseating the connection for the touch pad and try installing the synaptics drivers again?
T61: Core 2 Duo T7500, 4GB, NVS 140M
W520: Core i7 2670QM, 8GB, Quadro 2000M
T41: Pentium M 745, 1GB, Radeon Mobility 7500
Rest in pieces:
R40e (dead hdd controller)
T43 (Smashed by ex)