tong,
Your problem is that you are relying on your shoulder action to square the blade.......unfortunately it is not foolproof, as you have found....
Work more on changing your attention during impact to focussing on blade position (relative to where it was at address, if you have set up and aimed correctly)...Try and match your blade to where it was at address...
This involves having a flexible right hand, which your shoulders follow, rather than the other way around (to adjust blade angle to the correct position on the forwards stroke to make it correct at impact)....