Poor performance

Hello, I do not see any codes when I check. Under normal driving conditions it performs well enough. But sometimes when I am passing on the highway the engine kicks in then right back out again and it feels as though sluggish. If I come to a rolling stop then proceed and give it lots of gas it will feel sluggish. Winter time now, I had spun the tires in the snow a couple of times. When I do the engine revs up tires spin I let off and tires stop spinning but engine keeps revving as though transmission is slipping. I have thought of replacing the vehicle speed sensor but seeking advice on maybe something else wrong. I had just bought this truck over the summer I do not know all that was done to it. I only know a cold air intake was added as well as a throttle body spacer and was told a chip was put in. I do not know if the guy tuned it himself afterward. Is it possible the previous owner over tuned the shift pattern on the transmission. As I mentioned it does run good under normal driving conditions. I only notice these things when I step on the gas moderately hard.