I use the keyboard, not the mouse, as it's much faster. When I open the Purchases Journal, I tab from Transaction (Invoice) to Payment Method. If I pick Cash or Cheque then tab to Vendor, Sage automatically reverts to "Pay Later". This necessitates back tabbing twice to change to the correct payment method. The same applies to the Sales Journal. It would be much more efficient if the Payment Method stayed where it was put.
Have you tried using the 'pushpin' next to the payment method? This is supposed to make the payment method stay where it is put.