Road Cycling - Help! My freewheel is slipping

I put a USED 6-Speed Suntour freewheel on my USED Bianchi road bike. I love it! But.... Under load, the freewheel sometimes "slips" or jumps gears. This just won't do, as it usually happens when I'm starting out from a stop light trying to get ahead of traffic. Any thoughts as to what might be causing this? Is my chain too tight? Is my freewheel worn? How can I tell?

I'm using a Shimano eliptical chainwheel also, if that makes any difference.

thanks! Loke

Sounds like the teeth are too worn for the chain.
A chain will gradually wear out and get longer (called chain stretch"). The longer links will wear out the teeth. When you try and use the freewheel with a new chain, the teeth dont match up, so the chain slips. Its time for a new freewheel. If your chain is not new, replace it the same time as the freewheel.
If you replace your chain before it is too worn, the freewheel will last a lot longer.

Perfect advice above.

Also check your chainrings for wear. Shimano Biopace is really old stuff by now I'll bet that even under light use they're worn to the replacement point.

Thanks for yer help.

I'm looking into switching over to a Cassette system and possibly updating my chainwheel as well.