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???

Asked by for the 1999 Pontiac Sunfire
start with a tune up and fuel filter