Try replacing the fuel lines.
Mine, started doing the same crap.
Around town, in the garage, ran like nobodies business.
But, when cruising over say, 40 mph, the hot air coming off the engine would get the old black rubber fuel lines warm, and soften them up.
It drove me nuts. Then this.
Stuck on the side of the highway I noticed the fuel lines had collapsed.
I had pulled the fuel line off the petcock, thinking clogged screen.
Fuel everywhere.
WTF I thought.
Then I noticed the fuel lines had
sucked in on themselves. I actually watched them kinda pop back to round after about 10 minutes sitting there. The constant sucking of fuel along with constant heat from the motor, along with weak or worn out fuel lines was the issue.
Replaced the fuel lines, paradise.