so i think i figured it out.
i opened an old CPU intense project with no problems, so i was trying to figure out what the difference might be when i realised that for this project i bounced all the tracks at once using cubases batch export option.
it didnt, and still doesnt, make any sense because i dont think it adds any extra info to the file for batch export (at least it doesnt mention it in the manual), but this is the only thing i could think of.
so next i started a new project and put the same amount of roughly the same length tracks, which had not been batch exported, in to a new project. the problem was still non existent.
so i went back to my project which i batch exported my mixdown stems from and bounced them one by one using solo, like i always did before, then started a new project with them.
the project in theory should be exactly the same, but there was no stuttering, only the normal quick spike when i press play.
so it would seem that it was one (or both) of two things. i either had a problem with that project that i am unaware of, or using the batch export caused the problem. im not sure how batch export could do this, but i will not be using it for whole projects any more.
funny thing is i was laughing at myself because i knew batch export existed, but couldnt figure out why i wasnt using it
theres a reason it takes a fraction of the time!
it seems to be all good for now so thanks everyone for the help.