Bicycle Mechanics - questions chain skipping under load

I have a 1985 Schwin Sierra. Nice early Mtn Bike. All original condition (chain and drive train never replaced,

When going up a grade , I noticed the chain slipping at the rear sprocket the middle and second to largest of the 5 rear cogs. This was on pavement and I wasnt really grinding. On the other hand, I am a Clyde at 220 or so.

My questions

- I understand this is typically a sign of a worn chain. Correct? Are there any other possible problems with these symptoms?

- Is it likely I need a new rear freewheel as well?

Finally, is it likely that also need to replace the front chain rings? Doesnt slip there.

I ask because a new chain and freewheel seem pretty easy and low cost, but looks like new front chainrings might be more costly.

Lube your freewheel (http://sheldonbrown.com/freewheels.html#lube) and see if that makes a difference. You can measure the chain (http://bicycletutor.com/chain-wear/), as well.

A measurement of your chain will probably tell you what you need. Using the 12" pin to pin measuring test, if the chain has stretched less than 1/8", you may only need a chain; if it has stretched over 1/8" it may mean a new drivetrain. If you usually ride in those two gears, it may be worn freewheel. Visually it's very hard to tell if a freewheel is worn; the key symptom is it starts to skip..Even if the old chain is worn over 1/8" I would install a new chain as you will need to buy one anyway and if it stops skipping your good to go.

Usually you see skipping on the smaller sprockets not the larger ones. That's because they're typically used more, and with less teeth are more sensitive to chain stretch. I've start by assessing the chain's condition. Measure it for stretch. It's very rare that chains stretched less than 1% (1/8"/12") skip (assuming it's the original chain), so that would give you some idea of what you're looking at. OTOH if the chain is stretched about 2% or so, you can be pretty assured that the cassette is also toast. As for the chainrings, usually wear enough to skip is clearly visible as a sharktooth profile, so if the teeth seem symmetrical you're probably OK there.

Other possible causes of skipping include stiff or damaged chain links, bad RD or hanger alignment, and RD upper pulley riding to close to sprockets (makes sense since your problem is worse on larger sprockets).

Assess the chain's condition, and eyeball the pulley/sprocket clearance, and that will give you some ideas without spending money on trial and error approaches.

BTW- lubing everything as Zippy suggests is cheap and can't hurt so you might as well.