You're hearing the warning chime for the catalytic convertor overheat alarm. It has 2 levels: caution and imminent danger. The beep-beep is the caution level, the constant chime is drastic overheating. The LIE computer will cut-back the fuel injection system at this point to prevent overheating. This is a non-trivial issue and not to be taken lightly.
The cause of this is most-certainly either an ignition problem (likely) or fuel injection (such as stuck-open fuel injector) less likely. Please see http://www.lamboweb.com/Repairs.htm for much info on this; I would begin by checking both your ignition/plugs as well as the injectors. See the repair site for specifics
