Exported video from clips with different fps keeps lagging

Sorry if this is answered elsewhere. I am pretty new to Adobe Premiere, but until now I managed to work it out pretty well.

I made a video starting from clips shot with different fps. Some have 29,97 fps, others 30,01 59,99 and 60,00. Just two of them have 60,01 and 60,05 fps, I don't even know how I ended up with these. When I created the project, I didn't think much of it and used whatever preset Premiere was set on. Since the exported video kept lagging, I changed whatever preset I could find: I put 60 fps as the sequence's frame rate, I changed the timebase and display format to 60 fps. The export is set to 60 fps. However, as this did not work, I tried to find the mistake by exporting single clips with different fps creating a nested sequence for each, and they all lagged. I tried to export simply the credits sequence, which I created by setting it to 60 fps, and strangely enough each time I open it it lags differently: sometimes it moves smoothly, other times it lags just slightly, others it can be very evident. I don't know what to think of this, maybe the file is corrupted, but I doubt the problem lies in the player since other video in 60 fps are fine. So I guess there is a problem I am not seeing with the sequence or with the export settings?

Sorry if this feels confused. I worked a lot on this project and I would be very disappointed if I had to throw it all away. Thank you very much