I agree with Woohoo about encuentra24 and craigslist (except the part about pensioner focus; not sure what he meant by that).Craigslist doesn't have much, but it tends to be younger and more techie (though I don't know what field you're looking for).
Others:
http://www.epasa.com/cgi-bin/cf.pl
http://pa.clasificados.st/
http://clasificadosweb.prensa.com/search.asp
http://www.mercadolibre.com.pa/
http://groups.yahoo.com/group/PanamaUnclassifiedAds/
Generally, however, Panama being what it is, most info about jobs and internships is transmitted via paper or word-of-mouth -- not electronically. Therefore, if you specifically want something with the U of P, your best course will be to make friends with several people there...