low idle/shuddering

i've got a '92 two-door that's been shuddering BAD for a while.
for a start i tried some fuel injector cleaner but it hasn't seemed to have done much good. it doesn't always idle like that though. it seems to vary.

any suggestions anyone?

additionally, where the HELL is the oil sending unit located?
i've had the part for months and i cant figure out where to put it and Haynes hasn't helped me at all!!

Looks like it is located on the front side of the block facing you if you were standing in front looking at the motor. Line up dead center of the car and see if you can see it. Right below the where the top rad. hose goes into the block.