Lag when using virtual instrument...

[onebigtrip]

Today i ran into a problem using Reason 4 with PT that i haven't had before. I have a pretty full production in Reason that i want to add vocals too, but when i play the song through PT, Reason lags on every down beat. in other words, i have a 4/4 beat going, and when i come around to the first beat again, it kind of just lags for a split second - so the whole song is completely unfluid.
i use Reason often with PT to juice things up with synths and drums samples etc and have never had a problem with lag when sequencing everything in Reason and running it through an instrument track in PT.
my first instinct was that my computer was overloaded, but i find that highly unlikely as i have plenty of RAM and a dual-core 3 GHZ processor, and the CPU meter says im barely using anything.

this is preventing me from finishing my track! i just bought reason 4 and havent used it much with protools yet, so im wondering if protools 7.1 and reason 4 are having trouble getting along...


any ideas or clues would be much appreciated!