car missing out when i get to 60mph on 1999 Pontiac Sunfire

my car set for about a month and i had the heater core replaced due to anti. leaking in the passager side floor board so when i got it back that was fixed but now when i drive it i notice it at about 55 60mpr the car kinda jerks like it is missing out and then if i give it more gas or let up on it it quits and then it might be ok for awhile and then it will do it again ..can anyone help???

1 answer
start with a tune up and fuel filter