In any case, if your engine has more than 70,000 miles on the timing belt, it needs a new one at once. HOwever, the slip you experienced is not supposed to have damaged anything as this engine is non-interference.
Going past 70,000 miles on the belt is looking for being stranded in the middle of nowhere...tested by many...some info:
http://www.geocities.com/lmp4203/kia...TimingBelt.zip
Now,... having no torque below 4000 ...is just like mine. I'm sure you mean it is worse than before....any trouble code set?