Strange timing gremlins

suddenly severe hesitation brought me to dive into that motor thing deeper.
After a stop i suddenly had no power after all. That thing hardly managed to climb little hills.
The quest began (and is still on...)

Motor needs:
Timing
Fuel
Spark
Air
Compression

Timing:
I suddenly lost 10° of timing with the dizzy still firm on its place (used to be 10° BTDC, now was 0°)
Adjusted that - no luck.
Checked for sloppy timing chain and found it about 12° - so I decided to replace it.
in the first try I used too little sealer, so I had a massive oil leak in the front.

The second time I used black RTV and the oil went away. Timing chain play was now about 4-5°.
Still hesitation and no power.
Idles great so no major issue with the chain now I think.

Spark
New HEI-Dizzy - same thing.
Initial timing is 11°. Vac is working. Centrifugal advace also kicks in.


The idle gets worse with every spark cable I remove. So I assume every cylinder gets its spark.
Spark plugs regapped from 0.050 to 0.040".

Fuel:
Switched in a spare carburetor (new Motorcraft 2150) - same thing.
New fuel filter - same thing

Air:
Armed with a can of carb-cleaner I went for vac-leakes. Found nothing.
I suppose the intake is firm - check for that later.
Vac on manifold is about 17.7 inHg. Falling when I rev the engine and coming up again - so no clogged exhaust.

Compression:
Yesterday I did a dry compression test using a bad fitting tester and without opening the throttle wide while starting. In fact I got some pretty bad results.
1 - 90
2 - 90
3 - 70
4 - 85
5 - 90
6 - 80

This makes no sense, so I repeat this test dry and wet with WOT and a better tester.

so fucked.