Rough idle, no power, loss of gas mileage.

Recently picked up a gem, 89 p/u v6 4wd 137k. Ran great for 3 weeks decent mileage good pulling power no major issues. Rained for a couple days and I was hitting a couple puddles along the road when I noticed a small miss at load (low rpm's only) and a little rough at idle. No big deal I figured little water where it shouldnt be. Problem is getting alot worse and has been dry for a week or more now. Truck starts fine no miss at high idle then when it kicks down starts missing doesnt stall just runs rough. As you drive it the problem is hit or miss with no real pattern, may miss bad at pull out push in clutch tach it up a little and then power comes on strong. Do alot of highway driving and when it acts up its as if you are flipping a switch (with throttle in constant position) power comes on and miss goes away. Changed wires, cap, rotor, and fuel filter. Any help would be appreciated no light and o2 sensor under cab is not in pipe I understand this would be your first suggestion however truck was like that when I bought it with no problems other than chek engine light coming on sometimes. Oh yeah ran some lucas cleaner through it no help.