You could disconnect the front panel ribbon cable and use remote control over GPIB or RS232 and see if the DMM behaves correctly and proof that the problem is in the front panel only.
I like your advice!!!
And according this, I took a front panel from second unit!
![Grin ;D](https://www.eevblog.com/forum/Smileys/default/xgrin.gif.pagespeed.ic.QVVz6XIT20.png)
And First front panel connected to second unit... Result: Second unit work well with first front panel, But first unit NO, the problem still exist on the first unit. So:
Front panel absolutely good, front panel Connection good as well (I clean and checked it well enough). And problem does not appear when I touch front panel flat cable.
Seems problem somewhere on the board.