australia

just bought a 1992 ls400 with 93k. ran great for first 1000k, but put 91 octane instead of 93 and now i have a stutter at about 65mph after running engine for a liitle while(30min.). added ber. gas treatment but no help. have not added any new gas yet, but need to soon. is this an injecter challenge, bad gas,filter.HEEELP! engine idles at 800rpm, but one time it stuttered and dropped for a second; just like it does at 65. overall acceleration still great. thanks for any ideas.