I have a 1972 SBC 400 I put in my boat. When I first start it, it runs and starts fine, but after I go run it for a while it turns over very slow and acts like the battery is low. I checked the alternator and battery and they are both fine. If the boat sits for awhile it starts up no problem until I shut it off again. This is the second starter I put on it, trying to correct the problem. Please give any suggestions.