Another front friction shifter problem
Friction shifters eventually need to be taken apart, cleaned, and re-lubricated to solve the problems you mention. Be very careful to keep the parts in order. You'll find advice on the above mentioned other thread not to lubricate them, which is wrong. All the surfaces should get a thin film of oil or grease, ideally something that doesn't harm plastic presuming your shifter has plastic washers, as most do. Using a mild or medium strength Loctite is optional but is a good further safeguard against slippage, and won't keep you from loosening them on purpose. Some people use beeswax as a substitute.

The plastic washers inside can degrade, which wouldn't be surprising on a 30 year old bike. If so you probably just need a new shifter unless you want to get crafty and find or make new ones.