MD88 CREW HAD ALT EXCURSION.

Narrative

WE FLY; DEVIATING AROUND TSTM AT NIGHT; WITH AUTOPLT COUPLED; USING BOTH RADAR AND VISUAL MEANS TO IDENT CELLS. I WAS LOOKING OUT OF THE WINDSHIELD (FACE NEARLY PRESSED AGAINST THE GLASS; SO I COULD SEE BETTER); AND WAS CTLING MY HDG WITH THE HDG SELECT KNOB ON THE FLT GUIDANCE CTL PANEL. THE FIRST INDICATION THAT WE HAD LEFT FL310 WAS FROM THE ALT ALERT DEVICE; CHIMING AT 250 FT BELOW SELECTED ALT. AT THAT POINT; I DISCONNECTED THE AUTOPLT AND CLBED BACK TO FL310. WHILE WE'RE NOT SURE WHY THE ACFT LEFT ALT; I THINK THAT I MAY HAVE INADVERTENTLY BRUSHED AGAINST THE PITCH CTL WHEEL; WHICH IS ADJACENT TO THE HDG SELECT KNOB ON THE FLT GUIDANCE CTL PANEL. OFTEN; THE ALT HOLD DETENT ON THIS WHEEL IS LESS SECURE THAN ON OTHER UNITS. THIS INCIDENT COULD HAVE BEEN PREVENTED HAD THE PITCH WHEEL DETENT BEEN MORE SECURE. ALSO; ONE OF US IN THE COCKPIT SHOULD HAVE BEEN WATCHING THE INSTS AND THE FLT GUIDANCE CTL PANEL. INSTEAD; WE WERE BOTH LOOKING OUTSIDE AT THE CELLS. IT WAS DUE TO THE ALT ALERT DEVICE THAT I ONLY DSNDED 350 FT BELOW ASSIGNED ALT.