B&S 5hp is driving me nuts

Hi, this is a helmet scratcher....starts good, runs good both at fast and slow idle. After cutting grass for a while the engine starts surging with even just a bit of load, then surging gets worse even at standstill. I have to keep it running by continually pressing the primer bulb. It's like when the engine gets hot it bogs down. But this is at fast idle, at slow idle the engine runs fine. Here is recap of what I have done so far; newer fuel and air filter, newer plug, cleaned fuel bowl, new fuel. I also replaced the drive belt but that shouldn't play into this. What am I missing or what to do next. Appreciate the insight. B&S 5 hp 4 stroke. Thanks