1987 Pontiac 6000 sputtering

Engine Performance problem
1987 Pontiac 6000 6 cyl Front Wheel Drive Automatic 100.75 miles

Two part question:(1) When driving from 0-25 nothing, but at 30-35 it starts to sputter, at 40 & above it stops any suggestion.(2) When the gage is on one fourth of a tank and I start to brake then the gage goes to e and back and the vehicle shuts off it can be restarted immediately, on a full tank it does not happened. What help can be done. Thank you.