Purchased a used boat last week. When the previous owner started the boat for me I noticed it seemed to crank a little slow, but didn't think much of it at that time..and batteries were on a charger. A couple days later I tried starting at home and would barely turn over but after several attempts it turned just enough to start. I thought the batteries might be low, put them on a charger and tried again the next day. This time it wouldn't turn at all and just click once when turning the ignition. Knowing the batteries are charged, and the prior events of slow crank turning into no crank..I concluded it was a starter problem, so I installed a new one. Go to start and "click". I then jump wired the starter and it fired right up. Shut it down, went and turned ignition and it fired right up. So I trailered to the lake, went to start, click. Jump wired it and headed out. Ran for a while, shut it down, shortly after turn key and fired right up. Headed to a different spot, fished for maybe a half hour, turn key..click. Jump wired it and headed back to the ramp. Any suggestions to remedy the issue? I've checked terminals for corrosion and everything seems clean. 2nd issue. I am getting a popping or backfiring starting around 2000rpm. It starts easily and runs smooth up to that point, and can rev beyond that in neutral without issue...only occurs under load. One friend suggested bad plug wires, another bad gas. Any suggestions? I will be installing new plugs, wires, cap and rotor this week. Boat is 1984 22' Pursuit with Volvo Penta aq225d.