'87 Spider: Intermittent power loss under load

["Hillesheim, Jim" <jwh@xxxxxx>]

I need diagnostic help. Sometimes the car will act perfectly normal, going
along for maybe say 10 miles, revving freely to the redline, and then start
acting up for maybe the next five miles. When in its acting up  mode, it
will run normally under steady or part throttle, but given the least bit of
throttle, that is, when there is the least bit of load on the engine, and
there'll be a definite and immediate drop of power, as though it were
running on less than four cylinders: lift  the throttle just slightly and
its back purring right along on all four cylinders again. The car starts and
idles just fine.

Here is what has been done to the car so far:
Although it does not feel like a traditional ignition miss, a new set of
Bosch high tension wires have been installed, as well as new  NGK spark
plugs. Both timing sensors on the flywheel  housing have been cleaned.
Air intake hoses are all tight and secure; the fuel pressure regulator has
been replaced; the fuel filter has 2000 mi. on it.  At the suggestion of
Probst in his Bosch fuel injection book, I disconnected the alternator lead,
to eliminate the possibility of a bad diode fooling the fuel injection
control unit.  It made no difference.  I've run one tankful of gas with STP
fuel injection cleaner in it, and I'm now halfway through a tankful of gas
with a big dose of Techron in it.  So far, don't notice any difference.
So there I am.  Where should I look next?
Cheers,
Jim Hillesheim
Lawrence, Kansas
'87 Spider Veloce
'58 Giulietta Spider
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