ok well i have just replaced my intake manifold gaskets and also my plenum gasket and done a tune up but i just recently (like 1 day later) got a misfire or something going highwayspeeds when it goes to drop down into the low idle crusie and only after it has been driven for 15 min or so. any suggestions what it could possibly be?

 

Moved your post to its own thread, since it may or may not have anything to do with the plenum gasket. Welcome to DF.

 

Did you check or have someone check the cat, to make sure it's not clogged or partially clogged ?

 

no not yet but i will certainly check that.the wierd thing is it runs fine other that that symtom

 

Yep, clogged cat.