Rain and autopilot unstable

I have an S-tec 55 in a 1999 Piper Archer III.

Last weekend I was in moderate rain in a climb out of 2000 feet (900 AGL)
heading for 6000 feet when all of the sudden the nose pitched down.

I pushed the two drains located in the cockpit when the event first occured and
it did not solve the problem.

I disconnected the autopilot and handflew it. When it was safe to do so, I
engaged the autopilot again, and again, it would either pitch up or down.

Later in the flight when the rain had stopped, it worked as usual.

Any ideas?

Mark