Does it keep doing it? Or was it a one time thing? I guess that answer kinda determines the actions.
But without much information, I know if this was happening to me, my first gut reaction would be to check the intake to see if anything got loose causing a leak. An air leak can cause these things. I confess I haven't installed an intake on the Supra...But i have in many other cars...and sometimes if something isn't sealing correctly this can happen.
if your mechanic has it he should be checking for things like vacuum leaks, and things like compression testing. A cylinder misfire could be a ton if things, you need to narrow it down.