What would cause my truck to not have much power and the oil pressure gauge act erratic

I have a 78 K10 with a 350 in it. When i first bought it, it would cut out and die when you put it under a load (accelerate quick or going up hill). I changed the plugs and wires which were really bad and that problem went away. however, it still doesn't have much power when going uphill or with quick acceleration. I've got an 82 K10 also with the same motor in it and it has way more power. Also, when I first start the truck up the oil pressure gauge reads 60 and once the truck is warmed up (after maybe 15 mins of driving it then it drops to 30) and then it drops to 15 to 0 and will continue fluctuating from 15 to 0 unless I come to a stop. If I come to a stop with the motor just idling it will stay between 15 and 30. It does leak a little oil but is not knocking or ticking. Any help would be appreciated and thanks in advance!!