Hi Larry. The problem was not with 90sec subs. Its during frame and focus. Short exposures (ie: 1sec, 2sec) and continuous for frame and focus. Now you're point regarding adding another USB hub makes sense. I currently have everything running through one hub. I didn't have problems this way when using a DSLR but it appears with the ASI1600 I perhaps can't use just one USB hub. I will try adding another hub and report back.
Thanks for helping Larry!
Shawn