The only 1700mah batts that can handle 12-15A+ that I know of are the 1700AUP's...I suspect you have either AE's or AU's...probably the AU's...theyre only rated for 8-12A...they have a tendency to have problems right about the mark youre talking about with the 9x5, which explains why the 8x4 works. Here's a better motor simulation program.
www.motocalc.com You get a free 30 day trial...MUCH more options as far as motor, esc, batteries, props, speed controller, etc. More accurate too. If you really want to be able to run full throttle with the 9x5 you're going to need to get better batteries...try KAN 1050's...can handle the amps youre talking about and then some...can get em for $1.25/cell at
www.battlepack.com Look in the "Sale" section. Your run times would be a bit shorter but you'd be able to go full throttle without a hitch...they also weigh .4oz less per cell so your ship would be lighter...if you want to fork over some money though, go for the 1700AUPs...they weigh the same as the ones youre using now but can handle 25A. Let me know if ya need any more help...always willing.
Joe