it sounds like its time to replace this microwave. being 16 years old the parts wont be available to repair this not to mention the new microwaves are more powerfull and cheaper than fixing this, you probably have a problem with the wave guide or a problem with the magnetron itself, the other issue is the repairs will reqiure a service tech, heres the gorverment warning on this,
Consumer Product Safety Commission
Electrocution Hazard with Do-It-Yourself Repairs of Microwave Ovens
CPSC Document #5061
The U. S. Consumer Product Safety Commission (CPSC) warns against do-it-yourself repairs of microwave ovens. Each year, on average, four people die from electrocution trying to repair their microwave ovens. Microwave ovens use high voltage which makes it particularly hazardous for consumers to remove the cover and touch electrical parts. The possibility for electrical shock from a microwave oven still exists even after the oven is disconnected from the power source. Should certain internal devices fail, even unplugged ovens can cause serious injury.
The Commission strongly recommends that consumers have professionals repair microwave ovens because electrical appliances
can be hazardous if consumers expose live electrical parts.
Removing the cover of your microwave oven and touching an electrical part can kill you.
Have a professional repair your microvave oven
I would at this point just replace this, hope my info has been helpful , thanks Bryan