1998 Chevy Impala seems to hesitate or surge

1998 Chevy Impala mileage: 70,000. Lately while driving my Impala it seems to hesitate or surge, it's hard to tell. It started happening first at speeds around 40-45 mph. Now when it happens, it is doing it at 60 mph and also when I then start to drive slower. If I accelerate, it stops but then can start again. It doesn't happen every day and only seems to happen when I've been driving a while. It feels as if something is not flowing through properly. I had a transmission flush done at 60000 miles; they did not replace the filter. At 63000 miles, the fuel pump had to be replaced. The fuel filter and screen were also replaced at this time. I've never had a tune up. I'm not sure what it causing the problem. Should I worry about it stalling on me?