99 tacoma help

Hello

first time ever on a forum, I am very non mechanical and this is for my son's (college kid non mechanical) truck. 1999 Tacoma SR5 3.4 4 x 4 automatic

A while back his truck was vibrating wildly when reached 62 miles per hour, saw a forum said tires need changed, tire weight may have been thrown... he needed new tires so went this route. When he came home the other weekend and first time back on highway really since tires same thing vibrated terrible inside and no power

We have gone through and replaced plugs and wires (factory 2 electrode), changed the coils, and replaced fuel injector #3. Before did all this codes were coming up for cylinders 2,3,6 and running lean. Also have ran sea foam and cleaned Mass Air Flow.

Well this cured the terrible vibrations and once get going runs like champ. Therein lies the problem now to get going is a chore - it sputters, stalled twice, backfires and pops. None of this it did before we did all the other work. But once get through this initial backfire, popping runs like a champ until come to next stop light or turn a corner then repeat the process of backfire, popping, and get going runs great.

Not sure where to look next,