sluggish

my 88 sierra, ok i will change this a little runs fine no miss no idle problems when you give some gas she moves very well , but going down the high way at 70 you jump into it and it dies down does not stall ,going down high way you turn on the lights and it cuts out but motor does not stall , any funky wiring problems that anyone seen tps sweep ok , starting & charging ok no codes, converter not plugged good fuel pressure just had complete tune-up includes pick-up and module


thank you for any help on this matter