An MD88 autopilot transitioned to an uncommanded vertical speed mode; accelerating and descending; when the LOC was armed for an ILS on a vector heading. The autopilot and autothrottles were disconnected to regain aircraft control.

Narrative

First Officer was pilot flying; Captain was pilot monitoring. Aircraft had a write-up on the VNAV portion of the FMS/Autopilot. Aircraft was on autopilot and on vectors for ILS Runway 1R on a heading of 280 at 5000FT MSL and 190 KTS. Approach turned us to 340 to intercept the LOC to Runway 1R. The First Officer turned the heading bug to 340 and armed the LOC. At that time the speed bug went to 240 KTS. Both First Officer and Captain saw this and Captain tried to put the bug back on the assigned speed of 190 KTS but the bug was frozen in place. Turning the speed knob; pushing the mach/speed buttons had no effect. At this time the Captain noticed we were also in vertical speed and the aircraft was trying to catch the speed on the speed bug and we had descended approximately 350 FT. The First Officer disconnected the autopilot and autothrottles; pushed the throttles up and climbed back to the assigned altitude of 5000 FT. When the First Officer disconnected the autothrottles the Captain regained control of the speed bug on the mode control panel. The appropriate speed was set for the ILS approach and the First Officer intercepted the LOC and made an uneventful ILS landing.