BMW: hestitation/transmission fluid, bmw 325i, knock sensor

Question
hey whats up?
i have a 1993 bmw 325i manual with 106,000 mi.  the car is in excellent shape with one original owner who maintained the car unbelievably.  i have been having a problem with the acceleration hesitating.  i took it to the shop and they replaced the upper knock sensor and tuned it up (new plugs, filter, ect..). it ran great for about a week and the problem came back.  what it does exactly, when accelerating, the car feels as if it were not getting gas. it accererates good till about 2000rpms then, it looses power.  if i pump the gas, the power kicks back in, but i have to slightly pump the gas. sometimes it will do this around 4000rpms, i can never tell.  sometimes it will not do it at all. this is in every gear, mainly second/third.   
the check engine light is on and i tried to do the self diagnostic, but it didn't work for me.

i know how the car is supposed to run and it def. has a power loss.  could it be the lower knock sensor?and if so how hard would that be to change?

also, where can i check transmission fluid?

thank you very much!
kenneth  

Answer
Kenneth, I left this off the previous email, but you have a manual transmission, so there is no dipstick.  All you have is a plug on the side of the transmission to fill fluid with, and a plug on the bottom to drain it.  Usually you want the level of fluid even with the plug on the side.  Most of the time if you take out the plug on the side, you can stick your finger in the hole and feel the level of the fluid.  This should be changed if not done before, the car has 100+ thousand miles...