Very localized low

I think I flew through a very localized low pressure area,
perhaps six or eight miles in diameter and two tenths of an inch
of mercury deep.

Last Thursday flying South past Kansas City, about 1615Z, IMC,
6000 feet, Kansas City baro 30.14 on autopilot (KFC200), altitude
hold. I was vectoring East of a very active cell with dots on
the WX10 as close as the 15 mile ring; smooth air, light rain.

About this time, things got strange. I looked back at the flight
director and observed about five degrees down pitch, the
altimeter remained steady on 6000, the VSI on zero. Vacuum OK,
kicked on the standby pump for good measure... pitot heat on
(despite being 14C), alt air. All no effect. About this time
observe airspeed climbing from 145 to 160+ which on this airplane
(C82R) strongly suggests about a 700fps decent.

Altimeter still locked on 6000, VSI on zero, encoding altimeter
indication on transponder and TCAD all say 6000.

Problem is, my ears agree with the climbing airspeed indicator.

Kick off the altitude hold and bring up the nose five degrees to
bleed off airspeed; immediately indicate a climb on the VSI and
altimeters go up to about 6150. I'm about to tell ATC that I'm
going to be bouncing around for a bit when I notice the climb
stopped.

After about another minute every thing stabalized back to
normal. Back to 6000, airspeed normal. In a few minutes I note
the Chunate altimeter is 30.10. The entire episode took two or
three minutes.

All of this happened on the very downwind edge of an active cell.
Since the autopilot altitude hold is driven by the static system
I presume that it was just following an unusually steep pressure
gradiant and my true altitude dropped by about 200 feet for about
six or eight miles then returned to "normal."

Any other suggestions?