Right idea by Anomaly, but the Sun and Forte cassettes are not 11-28. To get that same size, try Ebay. Here's the cheapest that I found, although the shipping's a bit high:

http://tinyurl.com/lmf5m4

Also try a few other retailers, if buying off Ebay makes you nervous:

http://www.niagaracycle.com/advanced_search_result.php?keywords=7-speed

http://aebike.com/page.cfm?PageID=30&action=list&category=556&type=T&searchtype=attributesearch&searchDropDown146=Shimano+7&search163=&searchDropDown282=&searchDropDown2=&searchDropDown145=&searchDropDown60=&searchDropDown205=&searchFrom10=&searchTo10=&gobutnsearch=Search

ADD: One more thought: are you sure that the new chain is the correct length? The last time that I changed a chain, despite counting links several times, I ended up with a chain that was one link too long, and that produced some inconsistent chain skipping when I really put pressure on the cranks. As soon as I took out one link, the issue disappeared. You might try that before buying and installing a new cassette.