Are you running any oil additives?  How is your oil pressure?  It may sound
weird, but my '85 acted the same way when I put a can of CD-2 in the oil to
help with intermittent lifter noise.  It would run about 20 minutes, then
lose power. Valves would be clacking as I pulled over.  I would limp home
with just enough power to get inthe driveway.  The cat was glowing red hot.
My guess is either the lifters were collapsing and causing marginal valve
lift and dumping unburned gas into the cat, or the additive itself cooked
out of the oil and bummed out the FI.  I do have low oil pressure to begin
with and the CD-2 probably worsened it.  It may have thinned the oil too
much when hot, I don't know.  After replacing lots of stuff, including the
AFC, an oil change cleared it up.
                                                   Jay '85 Westy