94 Truck dilemma

[EFISYSTEMS at aol.com]

Hi People,
         I have spent an entire day trying to diagnose a problem, to no 
avail.  When the engine starts it runs fine for approx 10 seconds,then starts 
running extremely rich.....I then rev it up to "clean" it out and block 
learns and integrators are reasonable (123-125) then gradually returning to 
idle all numbers stay within reason and again will idle fine now for about 
3-4 seconds then whammo rich again, I was fortunate enough to have the exact 
year and truck to start swapping parts with.....here is a list of what I have 
checked or changed....
Fuel Pressure      steady at 13 when problem is occuring or not
TPS voltage         steady, no intermittent highs or lows
Map Voltage        steady, no intermittent highs or lows......even tried 
another
Coil                     coil was tried from other truck....supply voltage 13+
Tach lead disconnected from coil to verify no shorting of tach
Throttle Body and injectors were exhanged for the other truck
BHMX (matching)computer chip tried from other truck
Computer tried from other truck
Distributor tried from other truck
EGR checked for leaks and even blocked off at one point to verify idle problem
Temp sensor checked on scanner to verify no intermittent jumps
Knock circuit checked on scanner to verify no intermittent jumps
Timing verified and verified 10 degree jump with rpm(wire pulled) still 
changed dist later
Spark plugs changed....compression checked ,all within 20 lbs of each other.
Another shop had already replaced cap, rotor, wires, and plugs... to try to 
fix problem.
Customer had just come from muffler shop and had new o2 sensor, y pipe and 
cat back system installed since previous shop had told him he had a plugged 
cat.
Stored data on the scanner of 75 frames prior to engine stumble and 25 after 
....looked at every frame and compared all data points to see what voltages 
had moved or even if vss had somehow gone crazy......
All this and no codes on the scanner (unless you let it idle poorly so long 
it sets the high map voltage code)
The above is not in the order of which things were changed...just trying to 
recall from a very frustrating day.
At the end of the day I got the idea it has to be some sort of a grounding 
problem and the only grounds I know of on the engine are the two small ones 
near the thermostat, the two on the passenger rear of engine(one goes to the 
chassis from the engine and one goes to the frame)and the main battery ground 
cable......All seems ok.....but maybe there's more I am missing here......I 
then unhooked the battery for a few minutes (last ditch effort) and truck 
started and ran normally this time for 7-8 minutes,,,not seconds..........One 
last thing is this truck has had lots of stereo put in it and lots of 
capacitors for noise......if anyone can give me a spot to look tomorrow I 
would sure appreciate it and get back to the group with my 
findings......Thanks in advance.
-Carl Summers