86' 300E erratic fuel gauge

First I would like to thank you guys for your help W/ valve guides. I have this problem with my fuel gauge, the first 1/2 seems to register fine , but once below that it's all over between 1/4-1/2 mark, when I get down to about 2-3 gallons, the gauge wanders between R-1/4 somtimes as much as 1/3, I have replaced the sending unit,seemed to help a little but not fixed. I have the Electric trouble shoot manual but can't really determine what could cause the gauge to move all over the place, any ideas? Grounds? bad gauge? I did"nt see a voltage reg. for the insturment cluster, I guess MB doesnt use one,like old American cars had.
Thanks again Chris C.