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"for some reason whenever my character upgrades like new level in ninja some of his stats go temporarily down..is it normal?"
Don't worry, your stats haven't gone down, the upper limit has gone up, and so the stat has to 'heal' up to the new level.

"anyone knows wheres switch for that gate theres gate at the stairs i cant find a way to open it tried choping with axe, ZO spell"
If we're thinking about the same door (you mentioned level 4, with the worms etc), down the corridor before getting to the blocked exit stairs there is a mummy standing on a square, on the other side of a pit. There is a sign nearby ("This is my prisoner, let him suffer" as I recall) - kill the mummy! Throw any item at him, or use an attack spell; when he dies, he releases a switch, opening the door. Warning: the area also then gets groups of worms wandering around, and there are few points where you can get past them - you'll have to find a place when you can fight them instead.

Dungeon Master is a great game! I know the upper levels of the dungeon better than the area where I live... strange though, as much as I've played it, I've only completed it twice. Second time was earlier this year, when I completed a personal challenge - finishing the game only using one character (I used Sonya).