Tony, the 7 cell 300's are a perfect match for the IPS motors. You should easily get 15 to 20 hours form a motor with those packs. The 8 cells might cut it by half but not to less than a couple of hours. I've used 7 and 8 cell 300 and 370 NiMH packs for about two years with no unusual motor burnouts. Am now slowly converting over to LiPolys.
Either something else is wrong or by luck of the draw you got two defective motors in a row.
Did you use the same gearbox? Is it turning free? Sometimes the spur gear will jam against the housing. What size prop?
As is being discussed the 9x7 is the proven best for normal outdoor flying.