Random Win 98 startup problem

When starting (or rebooting) Win 98 (not SE) on my computer (Pentium MMX, 128 MB RAM), the machine hangs up quite randomly now and then just a few moments after the introductory Windows screen has appeared. The only remedy is to press the restart button, after which the situation may repeat. After one or several tries everything is OK. What is the best way how to find out the cause?