Probably you have an N900 with HW revision greater than 2101.

My U-Boot is based on Matan's u-boot and this version, as told many times, has problems (i.e. hangs on starting kernel) with phones different from hw rev 2101

You can remove u-boot from your phone by flashing a normal (aka without u-boot) kernel, and only the kernel, with flasher-3.5 tool.

There are many guides which can show you the correct steps to follow