I have a 89 Merlin Ti which I got last summer. From day one, I found the bike reluctant to move from the small ring to large in front crank. Initially, I had trouble with the bike going up hills, so I went to a 'compact' 35/49 crank, still w.in the mfr. specs for the Shimano 8 spd. set up (Dura Ace 8000). I replaced shifters (N.O.S), the rear derallieur (now a Shimano Deore) and nothing improved, but I started getting a lot of cat calls from various shops implying the changes are to blame, even though the slippage was the same.
I am ABSOLUTELY certain the bike likes moving to the large crankwheel when the bike is going down a hill. That, of course, also shows up when they put it on a bike stand at the shop w. the front wheel pointed down. Soon as I get on even a level road or one with even a slight rise, the left shifter doesn't do squat, or occasionally clicks with the chain looking like a snapping summer garden hose, never reaching the wider ring. Very frustrating.
Apart from only using the bike in zero gravity, and assume I pedal as lightly as Spongebob chasing butterflies when trying to shift, what in anyone's guess is a reason why the bike shifts reasonably well in front when I'm going down hill, in any rear gear, but acts like it has no earthly connection when flatlands arise?