1976 Other Ford Models Missing on 4-7-and 8 when idling

1976 Other Ford Models V8 1200 h miles

Hi!
I have what I believe to be a 1976-79 351 modified 225 HP Ford V8 coupled to a 280 Volvo Stern drive in a 25ft. Glastron fiberglass boat. The plugs have been replaced new, the wires and the distributor and rotor new. When I run her at idle using a presure guage I have thrity pounds of oul pressure but it fades to twenty pounds after four or five minutes at three thousand RPM. A lifter click comes into her then on what appears to be nunber four and she runs rough and won't idle at all. Idle at a thousand to fifteen hundred again for a few minutes and the click goes away again as the oil pressure creaps up again to thirty pounds. If you step up the throttle to fifteen hundred and pull four eight or seven from the distributor cap you can just feel a little rpm drop as if they are trying to pull their weight. Try to force over three thousand RPM and she craks and misses slowly reduce throttle and she seems to run on all at three thousand for just a few more minutes again until the pressure drops and the same thing again. The cylinder head prussure guage I used showed even prussure on all cylindrers at about eighty five with one at about eighty. ( They guage was off by up to thirty pounds said the owner I borrowed it from ). Since the low one answers to pulling the wire with an RPM drop, I figure low pressure isn;t really the problem, or at least not all of it. The carburetor is a four barrel with a new kit and seems to work well. I tried gunk lifter unsticker, No difference, I tried Lucas oil conditioner and no difference. What do you think could be the problem or problems. She isn't burning oil or discoulouring it. She isn't carboning up the plugs or wetting them down. She is acting like low compression and shuts off when shifting into gear from neutral unless your quick to give her a dart on the throttle. Any advice you can give me would be greatly appreciated.
Thanks and Best Regards: Jim Meaney