After some days of Intellivision reverse engineering I finally found a suitable solution to fix the ugly AV mod problems and it is now sharp, has much less noise etc. 

As seen within the youtube video regarding the TUT tutorial the Intellivision delivers first hand a really good and sharp picture out of it's video logic. But the video clearly shows (and this was not mentioned at all in the long installation guide but obvious recognized by many people within several threads) the output contains noise/interferance from power supply and HF modulator. 

If you disable the HF modulator by adding a switch to the 12V power line you'll get much less noise but the picture is awfull despite that. Other approaches replaces the HF modulator completely or offer an additional circuit to work around but do not fix it.

It suffer from:

1. bleeding and highlighting of areas which are in contrast (best modul for testing: frog bog)
2. moving/rolling pixels (frog bog, Donkey Kong: building)
3. washed and unsharp areas due to the noisy lines (best viewable if you reduce the brightness dramatically then you will notice lines are moving from top to bottom).

Analysis:

problem #1: this is purely a problem of the Intellivision design, it seems the onboard components changes values if it heats up (specially after several hours the picture gets worser and worser within minutes). The onboard regulator has a too short range so 1000th inch move decides if good or bad picture there is nothing between.

problem #2: is due to the nature of AV/composite and the only way to get rid of it is to use another output format (like RGB,S-Video) or a clamp filter. Using AV always leads to borders across areas that's by design due to mix of signals. This rolling is most likely due to luminance signal range moving into chroma range.

problem #3: GND and power supply problem. Too much noise from given pin locations.

I've verified all kind of standard video amplifier (all mods are nearly based on the same circuits and concepts) and it seems it doesn't care which one you are using. Sometimes you'll get a better less noisy picture but even that picture is too bad in comparison with other consoles. After some tests it pointed out the Intellivision video signal is soo good that is doesn't really required a big video amplifier or conversion, even a few resistors are sufficient. The picture is always sharp, colorful and much better than any other consoles like Atari or Coleco Vision. The picture has too much contrast and reducing it leads to noisy lines. But the mod requires a switch to allow justification of brightness.

So first approach was to disable the 12V line on the HF modulator. Noise was now a little bit better. But as good modders never every breaks anything (design nor functionality) I reversed the change. Finally I moved the AV solder point according to the schematics and now it doesn't care if the HF modulator is on or off, there is no difference in signal anymore and old HF modulator still works.

I also re-arranged the locations of the ground and power solder points and picture increased dramatically in output. The noise is still there but less visible. The added brightness switch on the mod was set to ~80% (you will notice picture gets quite clear and very good until it immediately drops to old quality if you move the switch too far). And of course it is important how the motherboard switch is set. It seems it is fairly overdriven in its possibities so replacing it with a finer one saves a lot of time in adjustment. The old one requires to adjust within 1000th of an inch to get the best picture out of it and still allows to detect the video signal - else if it is on a wrong position there is no picture at all or it fades away after a while.

Now the picture is as good as on other consoles. Frog Bog is playable now and Donkey Kong or Deamon Attack has very good picture similar to the RGB mod output (sure the AV limitations are visibile).

To address that i've tried to get a FMS6141 up and running to filter the picture a little bit but finally it failed because it is unclear where this IC comes from and if it is still working or simple dead. I've tried to find a video filter with clamp filter integrated to get rid of problem #2 but it seems there are less products on the market these days. As S-Video is nearly dead from TV set producers point of view the IC production was also cancelled for all kind of components. Bad because these chips also contains composite features so it is quite hard getting anything suitable to do the job. If someone has a hint regarding such a filter let me know.

PS: I've altered the PCB layout using my own Video Amplifier (which is completely different to the standard one but less complex) and together with the Stereo Surround Mod it fits on a 1" by 2" PCB providing the best sound and picture experience ever on Intelllivision (not seen the output of RGB yet but as the instruction uses the same solder points as AV I assume it suffer from similar issues than problem #3).