Chain tension and tensioning a chain

tallsam This is slightly unrelated:
Today on my geared roadie I noticed my chain slipping when I stamped on the pedals. It actually happened going across a junction and I nearly came off. I just changed the chainwheel (old for old but replaced for newer old) to fix this problem but its started again so I'm thinking it could be the chain or even the sprocket.

I hate it when bikes cost money for the boring non-fun bits (ie not an upgrade or a bling part).

Sounds like the rear block (this is a geared bike, right?).

If it happens a lot in lower gears, but seems okay in higher ones it probably means the cassette is worn, rather than the chainring. Otherwise it may just be the indexing is out, and the chain is not shifting cleanly.