Ive had the same problem before but Xport3 fixes it.
I use the BU-353 also. I never have a problem with GPS data as long as Xport3 is running.
Code:
Make sure you split the port into one more than you need.
Example #1:
You use Centrafuse with the GPS antenna (total of 1 GPS application):
say GPS is on COM4
Have Xport3 split the GPS com port into 2 ports (COM6 & COM7 for example)
Example #2
You use Centrafuse, iGuidance and GPSinfo with the GPS antenna (total of 3 GPS applications)
say GPS is on COM4
Have Xport3 split the the GPS com port into 4 ports (COM6, COM7, COM8 & COM9 for example)
^^The above has ALWAYS worked for me^^