Hesitations and backfires, what's happening?

[92bonny]

Hello everybody,

For a while now, about a year, the engine on my bonneville has serious hesitation issues upon starting from a standstill especially. It seems to go, hesitate (completely seems to stop revving) than continues on all through first gear like that 3-5 times especiallly on hard accelerations all the way to redline. Now it does it at any speed except on the highway it happens only in rare cases but it's increasing. And it happens at low and high RPMS. Also on hard acceleration under load usaully the car backfires through the intake (at least a pop sound comes out through there) What's happening? Sometimes it won't hesitate but it's rare. Nearly every sensor and part has been replaced on the car. I was thinking dirty injectors or carbon build up? By the way the tranny shifts extremely smooth so I doubt it's the issue Thank you very much for your replies and for taking the time to read through this.
Car- 1992 Bonneville SE N/A 113,000 miles