After loading a fight in progress, the plane loses altitude while it loads everything and starts the engines.

After that, the plane cannot keep the assigned altitude and ascends/descends too fast (1300fpm for just 100ft of altitude).

Putting ALT HOLD on does nothing, even if I tell the aircraft through the FMC to make a step climb to a higher altitude.

VNAV does the same thing, and it happens with ASN off, as well.

I verified the hold thing (pressing on the ALT HOLD knob) is off, and double checked that I changed the cruise altitude on the VNAV page and rolled the altitude up to the new cruise altitude to make a step climb.

After loading a fight in progress, the plane loses altitude while it loads everything and starts the engines.

After that, the plane cannot keep the assigned altitude and ascends/descends too fast (1300fpm for just 100ft of altitude).

Putting ALT HOLD on does nothing, even if I tell the aircraft through the FMC to make a step climb to a higher altitude.

VNAV does the same thing, and it happens with ASN off, as well.

I verified the hold thing (pressing on the ALT HOLD knob) is off, and double checked that I changed the cruise altitude on the VNAV page and rolled the altitude up to the new cruise altitude to make a step climb.

 

This happens on some machines, but not others. One of the workarounds is to enter slew mode (press 'Y') until everything initializes.

After loading a fight in progress, the plane loses altitude while it loads everything and starts the engines.

Like Kyle said what you describe happens on some computers during initialization when loading and then flying a saved flight. Just let the plane do its' thing and after about 30 seconds all is back to normal. Don't be messing around with the MCP buttons during this time period or try to do a step climb.

Someone suggested to me that when you load a flight, you should simulate time to 1/2 the normal speed until your flight stabilizes.