[Javascript] Radio button Event seems to "lag" in IE

[Glenn E. Lanier, II]

I have a sample page (
http://www.lanierconsulting.net/js/radiotest/javascript1.html ) that works
wonderfully under FireFox 1.5. It allows you to specify an amount, then
select a number of years (listed as radio buttons -- customer requirement
vs. select). After years are selected, it calculates total payment and
displays it. So, entering 10 dollars for 3 years means total payment is $30.
 
However, IE works differently. If I enter 10 in the amount and select 2
years it calculates 10 dollars (1 year is pre-selected). Then, if I select 4
years, the calculated amount is 20. Navigating back to the amount field
displays the correct calculated amount.
 
I am calculating when the textbox and radio onChange event is fired (which
may be part of my problem -- is there another event to attach to?) and I am
determining the selected radio button by 
 
document.getElementById('nameOfButton').checked
 
for each year.
 
Any guidance/critique is appreciated.
 
--G
________________________________
Glenn E. Lanier, II | glenn_lanier at netzero.net