Scheduled transaction problem

[Ian Merrithew]

I've got a scheduled transaction that has seemingly stopped computing 
properly, it's my car payment.  Here is the template:

Ian's Chequing Credit pmt( 0.05900 / 12 : 60.00 : 21989.69 : 0 : 0 )
Loan - Principal Debit ppmt( 0.05900 / 12 : i : 60.00 : 21989.69 : 0 : 0 )
Loan - Interest Debit ipmt( 0.05900 / 12 : i : 60.00 : 21989.69 : 0 : 0 )

So that looks right.  But here's what's happened in the principal register:

Payment #1 315.98
Payment #2 317.54
Payment #3 319.10
Payment #4 320.67
Payment #5 320.67 (should be 322.24)
Payment #6 320.67 (should be 323.83)
etc.

I'm up to 6 principal payments in a row of 320.67, with the corresponding 
interest figure.

Any ideas why the calculation seems to be stuck on payment #4?

-- 
Ian Merrithew - ADM Systems Engineering
ian.merrithew at ieee.org

"If a man does not make new acquaintances as he advances through life, he will 
soon find himself alone. A man should keep his friendships in constant 
repair."
Samuel Johnson
English author, critic, & lexicographer (1709 - 1784)