Hi. I have a 91 lx 5.0 and had the same problem. I fixed vacuum leaks, replaced purge valve, replaced coil, tune up, checked timing, ran codes and fixed items referred to by codes. My car ran great except under wot. Got to about 3500 rpm and would break up real bad. If I eased off on throttle would run better. I bought and built car for hpde and having this problem not acceptable. Turns out problem was TFI (thick film ignitor) unit on side of distributor. I got MSD replacement unit from American Muscle because it comes with the insulating film required. I usually prefer motorcraft but they have bad reviews. Put this on and car runs great and problem is gone. You need to turn distributor to get access to the screws but very easy to fix. Need special tool so order that when order tfi. Will also need to reset timing since you moved dist. Not as rewarding as drinking heavily but car might run better. It was very frustrating for me because every time I would fix something car ran great until next road trip and problem was still there. After replacing tfi I have done 6 different events and problem is gone! Hope it works for you.