Engine problem - Seneca II

We've been having a problem with the engines on our Seneca II for awhile now
and I wonder if anyone else has any experience with it. The engines are
TSIO-360s and if we lean at cruise with about 75% power (typically around
32-in MP) or less to anything over about 1400 EGT (which is still ROP), the
manifold pressure will begin to drop off after awhile - sometimes after only
5 minutes or so, sometimes after 30 minutes or more.

It drops eventually to what would appear to be ambient pressure, as if the
turbo suddenly stopped providing boost. If the mixture is richened up the
MP will eventually return to normal - usually you have to go to full rich
until things return to normal and then lean back but not as far. Fuel flow
rates at the mixture setting that keeps this from happening are well over
what the book says they should be - as an example at 8,000 feet and 2300 rpm
with 32-in MP we need about 14 gph to stay below 1400 EGT. Lean the mixture
to below 14 gph, even to 13.5 and allow the EGT to rise to 1425, and the
problem will show up. The engines run fine when this happens - no
roughness, etc.

We have tried lots of adjusting and changing of parts in the fuel system -
about the only thing we haven't tried is changing the turbos because that is
prohibitively expensive - and we have always allowed 3 min of turbo spin
down time before shutdown. At first only one engine was doing this but now
both do it and have been for the last 1,000 hrs or so. Both engines are
still healthy with good compression in all cyls, so whatever this is it does
not seem to be causing any damage.

Anyone else ever seen anything like this?

BDS