I would actually suspect a worn cog and/or chain. I had a similar problem with an older bike and swapping on a new cog and chain made the problem go away. If you can, put the bike on a bike stand. If it shifts and peddles fine with no load but skips while you're riding it, then it's time for a new freewheel and/or chain.

Edit -- Well poo. should have read your post better. Probably not a worn cog if you put on an 8 speed setup. Did you also install a new chain?

I have a Suteki Mixte in my shed awaiting restoration. It's in nice shape -- just needs cleaning and TLC. Not many of them around!