power sterring presure problem

[kmakovac]

i have a 86' NA 944 with power sterring problem:
1.the power steering does not assist to much(i feel the right turn is harder)
2.sometimes when i turn the steering whell like a contra presure hapens and the steering wheel vibrate(car stop) and when i turn it he turn me on the other side

any ideas?thanks for every kind of help,kristian