power loss

[alumacraftmerc150]

My 2001 Mercury 150 Optimax started and ran perfect on it's first boating trip yesterday,but after a while the warning beeper came on and I lost all throttle power. It still idled just fine.It continued to do this every time I gave it the throttle and the warning beeper would not stop until I completely throttled back .I then unscrewed the oil reservoir cap and there was a instant vacuum suction into the reservoir. I tightened the cap , then throttled up the engine and it took off just fine, About every 1-2 minutes it would to it again and I continued to solve the problem the same way until I got back to the dock.Could anyone tell me what they think the problem could be or some trouble shooting that I might try! Thanks!