1) Performed swithnbait on several components - including relays, DME's and EML. Installed new oxygen sensors, spark plugs, and battery.
2) checked for intake gasket leaks by spraying the areas with some type of combustible fluid (not sure what he said). Said that if the engine began to run differently that you would find a leak this way.
3) checked fuel pressure and entire fuel delivery system
4) checked the IAT - not sure about switchnbait
5) not sure about checking the fuel injector solenoids... is there a procedure for this?
6) checked the MAF but did not do switchnbait
7) checked output of O2
8) checked the ECT
9) not sure about the spark sensor coil.... what is that?
10) checked the coil output when hot
11) not sure about what was done with distributor rotor... have to assume that since the plugs were changed this would have at least been inspected.
12) As mentioned performed switchnbait with EML and DME
13) All plugs were of the same condition when pulled.

I am beginning to suspect that maybe the spark plug wires may too old. The mechanic did not want to do a switchnbait with these for fear that he may damage the other cars wires. Seems they get fragile when they are old, and they are very expensive for this car ($280 for left bank and $340 for right). Is there any way I can test these (maybe resistance) or possibly a cheaper aftermarket version.

What do you think?

Thanks!
It is pretty tough from the thin details you have provided. It is a lot like the teasers before the main feature at the movies. What diagnostics have they gone through? What are the "other parts" you have referred to?
1) has the "switch'n'bait" routine been tried? i.e. switch components from left to right to see if the problem flips sides.
2) these beasts have a reputation for leaking intake gaskets that may not show up until everything gets up to temperature. Has the mechanic checked for that?
3) has the fuel pressure regulator been checked for a vacuum leak? for set pressure?
4) has the IAT (inlet air temperature) sensor been checked for good readings - switch'n'bait?
5) have the fuel injector solenoids been checked to see if one or more is staying open?
6) has MAF (mass air flow) sensor output been checked - or the switch'n'bait tried with the MAF?
7) has the output from the O2 sensors been checked (i.e. oscillating between 0.2 and 0.8V) - or the switch'n'bait tried with the sensors (switch connectors)?
8) has the ECT (engine coolant temperature) sensor been tested - one connection but contains a thermocouple for each bank?
9) if it is misfiring a bit as well, the spark sensor coil on #1 plug wire may be producing intermittant signals.
10) is the coil output strong when it is hot - may have poor spark energy causing incomplete burn (switch'n'bait - awkward but not impossible)?
11) same with the distributor/rotor - (switch'n'bait with the rotors)?
12) did they check both the DME's and the EML modules?
13) have you pulled the six plugs to see if it is affecting all cylinders or just one or two in particular?
As you can see, the longer you think about it, the more things come to mind - more information is definitely needed to take it further. This is an interesting one...let's hear what you can tell us.

'88 750iL

I have an 88 750IL that has been in the shop for the past 3 months. The car will starts and runs fine from cold... but after running for a while the left bank begins to run very rich. The mechanic has given up. He has replaced the oxygen sensors. Then he replaced other parts in the car with parts from a 750 that runs fine (including the engine control units). Still the left bank of the engine runs rich (seeming not to run at all). He synchonized the throttles also (evidently doing this on a 750 is a unique process).

So after 3 months in the shop, I have the car back with no clue how to get it fixed.

Can anyone help?