I think this is a problem faced by many! if you jump or hit jump in the part where you lose your weapons, you can neva jump again apparently. This is what happens to me all the time in that lvl.
I tried that but I could still jump. I will show a recording in a minute.
[YOUTUBE]http://www.youtube.com/watch?v=2q35h_l_dic[/YOUTUBE]
O M G Z ! NO LAG!