Rear derrailleur problem, and mounting pictures, bike ID?

Hello all.
this days I was cleaning, repairing, and mounting a road bike ( that came incomplete), I think it can be a Favorit, as I post some days ago....but I'm not sure now.
8 speeds.
The only serial number found is in the fork: 214.
?
any ideas?

Well the rear hub is a vintage Campagnolo, the chainring a Stronglight, and I have bought a pair of Sheffield pedals, and a nice Favorite 53 seat.

Well, I bought a cheap brake levers to start, and put the center pull brake to work. Nice.
I replace and grease the 4 bushings.

The derrailleur is a Shimano Skylark. So I bought a new chain, and they mount it and set it in the shop. All the gears, perfect.

yesterday I rode, and I have a problem, so I need your help.
When I apply force to the pedals ( in a rise), the chain jumps up, and comes down, in each pedal turn. a nightmare.
All the others gears don't work properly. when you apply force, it jumps!
When the rear weel is in the air, everything is nice and soft....
Someone sais to me that the chain is a large model ( not long) and this is the problem.

The cassette is a 4 gear, and has no play, and looks perfect.
I don't know if The rear derrailleur is bolted in the correct position.....
Any advice?

here some pictures

Thanks!

The chain slipping is what happens when you put a new chain on old cogs, even if they look good.
You could possibly fit a new 5 speed freewheel.

The front forks still look about 2 centimetres bent back. It is quite possible to pull them forward.

Keith

Re:

Hello Keith, thanks for your answer.
I understand, but seems so strange that it does the same movement on any gear....
talking about the fork, the tube on the frame is clean without marks of ¨violence.
how can I bend it? it scares me...

Could be a stiff link in the chain? If there is, bend it sideways a few time on the stiff link, this may loosen it enough to free it.