As indicated above. replace the caps. use the non vented radiator cap. make sure the recovery tank hose is good and the tank is filled to "ADD" when cold. (You're suposed to read the marks when hot....)
Also have the system pressure tested. make sure it holds pressure for at least 10 mintues. Even a pin prick leak spells trouble for Fiero. If it is leaking the #1 suspect after hoses is the heater core.
Assuming you don't have one of at least a dozen other problems....
The OE radiator size isn't your problem. It may however be full of sh_t. In what case I'd just replace the bloody thing. New Modine replacements aren't all that expensive. $179(us) I think someone recently told me for V6. You can get Modine thru many parts stores.
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11-Sept-01, The day the world as we knew it ended.