1980's 20hp Evinrude

[Steeger]

My 20hp outboard quite often dies when trolling at low speed
But always starts again when pulled
When this happens the ball in the gas line also loses it's prime. I have tried two new lines so I know it's not the gas
line. The outboard runs excellent at top speed
It is only when trolling that it loses prime and dies.
Any suggestions where to start?

[boats.net]

Could be an air leak in fuel hose connections, weak fuel pump or bad anti-siphon valve at tank(if has built in gas tank)