Bicycle Mechanics - Accushift Slippage

These are the original shifters so I've ridden them for 17 years. They are Suntour Accushift (6-Speed) and when I really lean on a gear (like when I go uphill), the right (rear) shift lever slips upward, moving the derailleur to a smaller sprocket. I tried taking them off and cleaning them, but I still get the same thing. They are secured tightly to the down tube. Do they need to be replaced?

Paul

Is there no adjustment to tighten up the friction mechanism? I know some friction shifters have this adjustment built in. If not, I'd say they are ready for replacement. Now would be an easy time to switch to a 7 speed freewheel and new 7 speed downtube shifters. Sunrace offers these pretty darn cheap: http://www.niagaracycle.com/product_info.php?products_id=509019

If you felt like a bigger upgrade and had the cash, you can shove a 130mm freehub wheel in your rear dropout and move to integrated shifters. Ribble has some great prices currently on Shimano groupsets including the new 10 speed Tiagra group. You can get a complete group for less than $450 delivered (highly tempting for me at the moment if you couldn't tell).

Are you riding with a accushift freewheel as well? Also, are the levers set to "F6" or "index"?

Joe, there is a wheel to tighten the mechanism, but if I tighten it any, it will be too hard to shift. I might think about your 7-speed idea, but I just got a new Suntour Alpha 6-speed freewheel and a nice, Royce 126mm hub, which kind of answers Puget's question. The shifters are set to index, and only slip when I am putting max effort into the pedals. I do not think it is an Accushift freewheel. I never knew there was such a thing.
I think I'll just replace these shifters. Thanks, people.

Paul