We have a challenger 200 amp 240V main service panel. It was installed as new construction in nov. or dec. of 90'. The main breaker has started to trip off on occasion and we have to stumble around and get to the panel to reset it. I was told the breaker was discontinued a long time ago. What is a guy to do??

Assuming there have been no modifications to the panel, I don't know why the main breaker would have been "discontinued" and if it is tripping it is obviously hooked up.
The main breaker could be malfunctioning or it may simply be doing its job--that is tripping because more than 200 amps are being drawn by your home circuits, which can occur even if none of the individual breakers trip. Other wiring and/or bad breakers in the main panel could also trip the main breaker. Assuming the main breaker can normally be reset without your having to turn some appliances off first, it could be a bad main breaker or a short or defective branch circuit breaker in the main panel. I am not a licensed electrician.

Main breakers rarely fail on their own. There is a better chance the panel itself is causing it to trip. A bad or burned buss is likely the culprit.

This is typically NOT something a DIY can take care of. I would call an electrician out to check it.

Electrician believes only aluminum wire used in house from meter box to main panel was not tight. He racked down on the nuts holding the feed from outside to the breaker, charged us $95 and went home. No problems since. Said we should have been tightening these since we built the house 17 years ago. Aren't those nuts hot?

Thanks... Bo Whalen

Yep. And that's why you pay a professional to do it. A misguided screwdriver can fly off and get really hot fast.

A more comprehensive/expensive repair could have been done at significantly more cost.
Remove meter or have it removed. Remove and clean wires. Apply anti-oxidizing paste on wires. Use Loctite 222 on threads. Have meter put back and have the seal put back on.

Locktite 222 is a low strength threadlocker. It works wonders.