A PA46 pilot reported an autopilot malfunction or possible runaway trim attempting to level at 4000 feet. Control was regained; although the trim problem was not corrected prior to landing at destination.

Narrative

Due to challenging conditions I performed a very thorough preflight and departure preparation. ATC cleared me to destination at 4000 FT. It became clear as I approached 4000 feet that the autopilot had something wrong and was going to fly through it. I selected (ALT) at 3950 FT but it continued to climb to approximately 4250 FT. I tried to disconnect the autopilot with the yoke switch and later at the off/on switch for autopilot to no avail. All this was while over powering autopilot and pulling back power and hand flying course. I told ATC that I had autopilot problems and they volunteered that I could land at a close by airport; but I didn't want to reconfigure my NAV.; since it was still programmed for destination. I was having lots of problem holding altitude. Nearing destination I got a hand off to a frequency that was impossible to hear clearly; so I successfully called Tower and told of my situation. Tower cleared me to land; which I did with no problem. I very much appreciate the help from ATC controllers and was happy to get on the ground safely. I believe that the trim on the elevator went all the way to the stop and became lodged there somehow.