Very low idle and dimming lights, Celica had a heart attack ?? Options

[Remy]

Hey all, this morning I was driving to work on the highway and as I was approaching my exit this truck started tailgating me I was like wtf ? So I switched lane and he followed me as I got close to my exit he was still right behind me so I down shifted and took the exit and lost him as I approached the stop lights and coming to a stop my car started idling down to 150-200 and my lights and interior starts to dim down really low I though the car was gonna die on me, it was a good thing my work place was just down the block.

So I got to work and turned the car off and went inside, after I got off of work I started the car and it ran just fine drove it home (took the small street though). At first I though the alternator died on me or was going out but I drove it home after work just fine no problems at all and good idle too...? My other though was maybe its the IAC/ISC that was dirty but why would it effect my car only on the highway when I drive 60-70mph ?? If it was the IAC/ISC then it should effect my normal driving on the streets too right ?? And my idle would be high/low too ?? Anyways post up what you guys think the problem could be.....

Oh and the car just recently got seafoamed through the intake manifold only.