Hi. I have a newish to me 1980s Univega SupraSport with Suntour derailleurs (12 speed). The following sequence occurs every ride or two:

- When going up hill in a lower/lowest gear, the gears will slip all the way to the smallest cog
- At this point, I am stuck in the highest gear and cannot down shift (continues to slip); I stop and tighten the d-ring on the shifter to tighten the cable a bit
- Everything is OK for a ride or two
- Repeat

Cable looks pretty new.

Before I start taking things apart (the downtube shifter? New cable?), I'd be grateful for some advice if this scenario clearly points to a cause.

Many thanks.