[TenTec] OMNI V Low Output With Certain Antennas Question

[OTAKEBI at aol.com]

I am at the end of my rope.
Can't seem to find an explanation for why I can only get 80 watts output on 
10 and 20 meters with my OMNI V when on a Butternut HF6V-X and my Butternut 
Butterfly Beam but on the same bands when I am on my G5RV I get 100 watts output.
I am going through an MFJ Antenna Tuner and SWR is less than 1.5 to 1.
Same Tuner and SWR on G5RV.
If I go to oil filled Dummy Load I also get low output.
I have an MFJ 259 Analyzer and it is showing the same SWR as the meter on the 
tuner and it shows a resistance of 50 ohms at the stated SWR.
If anyone can shed some light it would be great.
Dan/N4VET