Not sure

Take a hose to your battery and blast away any corrosion. Next remove the battery clamps and wirebrush each positive and negative along with any auxilliary connectors on the positive side, On the negative side, trace the cable till it comes to a bolt that holds it onto the chassis. remove that bolt, clean that connector, wirebrush the connection point hex nut and washer and assemble it along with the 1/0 cable leading to the motor block. All these connections must be sparky-fresh and be able to pull the 300 cold cranking amps. It is only this way we can be confident that the ECU is getting clean power not "brown" power which causes failures. comprendez?~

All GMC vehicles featuring OBD keep their trouble codes...I could be wrong, but if you hooked up a code reader, you'd see an assortment of failures past, and present.

http://www.jcwhitney.com/1974-2005-buick-lesabre/ignition-control-module/p2019009d198y1974-2005j1.jcwx...about $100 http://www.autopartswarehouse.com/shop_parts/ignition_module/buick/lesabre.html..a bit more, there are other suppliers too

You have NO check engine light? find that hard to believe...even so, let's try to break it down into performance issues. 1.air 2. fuel .3. spark 4. timing...apparently have a new ECU, but probably did not go basic, power from the battery like I said in the first entry....turns over round N round, so we've got the 300 amps cold-cranking power, Spark. could pull out a sparkplug, leave it connected to the secondary ignition, ground the body of the plug by wrapping it aluminum foil, connection to ground, turn it over and see if it sparks, there is a dandy high-voltage detection wand I used to have when I was a mechanic that'd glow when high voltage was present, but it got stolen one day~ You'd smell gasoline if you pulled out the sparkplug, so one may guess we've got fuel pressure, but still not sure where to begin looking on this puzzle...please drop me a few more clues~