How To Fix Timing In A Midi Track?

I exported a midi track from Sonar, and imported to a track in Reaper. (Actually I may have just done a copy/paste, and that could be the cause of this problem. But I dont remember cuz it was a couple three months ago that I exported everything all at once out of Sonar.) Anyway, the midi track in Reaper starts out ok, starts right on time, but the farther into the song I let it play the more the timing of the midi notes drifts out of synch. By the end of the song it is more than 1 whole beat off. Any idea what caused this? And is there an easy way to fix it? I'd prefer not to play it again if I dont have to as I'd rather spend my time recording new guitar tracks. it's odd because I did this same copy/paste with other midi tracks in other songs (from Sonar to Reaper) and they all play just fine. This is the only one that gets out of synch.

Sounds like the MIDI clip is playing at the wrong tempo. I'm not home right now to give more specific help, but if you right-click the MIDI item and go into one of its Properties boxes, there should be an option somewhere to set the item's timebase (what it uses as a reference). In this case, I'm pretty sure you want it to be Bars/Beats. If this doesn't help, try importing the file again. Reaper always asks me if I want to use the file's tempo or match it up to the project.

IMO, it's quite impossible to copy and paste a midi file from a software to another, cause they dont't manage midi the same way(internaly). the only safe way is to "save as a midi file" in the first software, then "open a midi file" in the second.