A LJ-35 Mach Trim Computer malfunctioned in flight preventing the aircraft from being trimmed at FL260 but normal operation returned after descending to FL230.

Narrative

We had climbed to FL 260 and were in the process of leveling off at our assigned altitude of FL 270. The First Officer was hand-flying the aircraft. As he started his level-off he indicated he was encountering trim problems with the aircraft. I took the controls and attempted to level the aircraft and also experienced the feeling that the trim was not responding correctly to inputs. I reduced power and as the aircraft slowed I was able to regain positive pitch control but during the process we deviated above and below the assigned altitude. We also got a Mach Trim warning. We radioed Center and informed them of the problem and asked them if they had had any reports of wave action (we were experiencing moderate chop during the level off) and they responded negatively. We then requested a descent to FL 230. I told the First Officer to engage the autopilot and we descended to the requested/assigned altitude of FL 230. With the auto pilot engaged we continued the flight with no additional problems. I discussed the problem with our maintenance department and they feel it is a problem starting with the Mach trim computer and they will be checking it and making any needed repairs.