AP Alt issues with Transall C-160?

Sometimes my AP just let the Transall sink until ground. Flaps are at 0 and speed was >200 knots resp. enough to increase altitude. Is the AP broke somehow?
And where can see the VS in the cockpit?

Maybe you had the AP in pitch mode?

AFAIK, the Transall doesn’t have VS. I recommend reading the manual.

I followed the orders, always the same as in this vid shown:

But sometimes I got the problem with AP and sometimes not. That´s what make me clueless. And in the manuals i could not find information about VS. Probably I have overseen them but I did not found them.

It has VS.

The problem I have now is not the same the one I described here. For some reasons I dont know it is a misunderstanding.

Now the problem is that when I activate AP it wont work as planned. e.g. I want 1500ft/min and get much more than 14000ft/min.

Have you used the pitch up / down switch on the autopilot panel next to the FMC to adjust your rate?
Its a rocker switch next to the start/idle levers