Check engine light 750iL

[boisebadboy2001]

Hello, I am a new member, my name is Ben. And I think I can offer help on here as much as I migt need help from others. I am a 28 year old mechanic who does all of the work on my cars, and about 230 other peoples cars (return customers) in the Boise Idaho area.
My problem is on my 1989 Bmw 750iL (e32) with the m70 v-12. This car has 128k on it and runs like new, except one little thing with the check engine light. Remimber the motor is fully rebuilt, same with tranny (a pain to rebuild!), it has both motor high performance chips, and tranny chip. custom 3 inch exaust cat back, cats are factory as are o2 sensors. no resonator, and high flow muffler. Motor itself is as stock, except the chips.
Now this problem just resently started, the check engine light flashes or stays on for a sec. then flashes when slowing down from freeway speed (70mph). Also does this when driven hard and in sport shift mode, when revs are higher then 4k then slowing for a corner, or a cop (lol). But doesnt only do it in sport mode. But only when letting off the gas at higher speed, or rpm. It seems to do it more when lower grade fuel is used, with 93 oct. and a booster it happens half as much, meaning less period of time flashing, or maybe just a flicker of the light then thats it, where lower grade fuel is used it may stay flashing for up to 10-15 sec. when slowing off freeway. Sometimes a few sec. steady glow then flashing. The gas tank is new (due to large dent in tank, wife and a curb hop....... Ouch), and so is the pump and filters. Lines have been cleaned, and all seals in intake have been gone over, includind injector seals. Mass air sensors have been checked, and throttle bodys have been tested. Everything seems to be working fine, except havent checked fuel rail pressures, and those valves, and fuel return line. Anyone have any other ideas, thinks i may want to look at. The only reason i have been thinking its a fuel problem, is the change when diffrent gas is used. Any help from you other 7er owners would be great as im stumped for the first time with this car......

[Antiburn]

If youve never replaced your 02 sensors that could cause a Checke ngine light. You should get a short test of the car and know exacatly whats wrong

[boisebadboy2001]

Yeah, sorry didnt put it in my post, but i have had 3 shops run tests on my car, and also a computer check of all systems threw a bmw dealer. I guess my car says theres nothing wrong, like the light isnt coming on at all. which is strange to me because even my dads 93 ford (POS), stores trouble codes to read with a scanner. I guess my car is just haunted, lol. It doesnt seem to show any signes that anything is wrong. As it runs great. Other then the shaking, and hunting down the reasons why (not uncomon for a car with this many miles). This is why I put a post in here, not because im another person who posts things that are easily fixed with a trip to the dealer or any other shop for that matter, but because i have a problem, and I am stumped, I think so is a fow other mechaincs in town. And the 02 sensors have been checked and checked again, taken out by a shop cleaned, and tested again, even tested on a bench, I wish it were that easy. Was just wondering if anyone else has ever had this problem on a car, hopefully a e32 with the m70. But any advice would be worth lots of thanks.

Thanks a lot.....
And thank you Antiburn for the reply....

Ben