Shimano 3503 makes multiple upshifts
 

This just started to happen after working normally. On upshifts (low to high) the rear shifter now shifts 2 or 3 gears at a time. All the down shifts are good. It is relatively consistent. From the lowest gear one click on the lever upshifts twice, then the next shift is good and then it upshifts 2 or 3 on the next click and then its good down to the smallest cog. On these bad shifts the shifter itself makes an uncharacteristically heavy/loud click and in doing so upshifts multiple gears at once, as opposed to making one click and the chain jumping cogs as one might expect with a bent hanger or with funky indexing. I have cleaned and oiled the shifter mechanism and checked that the cable moves freely in the housing. With the cable disconnected from the rear derailleur and me holding the cable with some tension i can count the 8 clicks both directions, however the upshifts are noticeably lighter and quieter than downshifts. With the cable connected and indexed then i can upshift across the cassette in 5 or six clicks. Any insight would be helpful. Thanks

The usual possibilities come to mind. Cable friction being #1. bent hanger or cage pivot is #2. Cable clamping mistake #3. Andy

Check inside the shifter to ensure that the cable is not fraying. It is much easier to replace it before it breaks than after.

If it's not the frayed cable, I'd guess one of the pawls in the mechanism didn't get hit with enough oil and is slipping only when under spring tension, and not hand tension. The upshifting mech may be on the underside of the works. Try removing the shifter, inverting it, and flooding it again. But check that cable first.

I will try this thanks. The cable is in good shape as I replaced it recently along with the loop of cable housing to the RD. As I mentioned, everything was working well. I didn't mention that this sudden issue came after I did my regular wheel swap - road to gravel. This is a regular swap for me - and not one that has caused this problem before.

Do you have a bare cable going along the downtube? hold on to it lightly as you shift and see if the cable is going slack. If it is then something is binding somewhere further down the cable.

If it doesn't feel like a proper amount of cable is being released, then it's binding further up the cable and possibly the cable is frayed up in the shifter housing. You want to fix that quick before the cable is so frayed it makes you cuss trying to get it out.

The cable is only exposed on the chain stay. From which point it slides smoothly in both directions. When i pull the cable out of the shifter then I see no fraying. I think i will just take everything apart, clean and lube what I can and try all of these suggested tests, otherwise I will begrudgingly look for a replacement shifter.

I tried everyones suggestions but cannot improve things so have ordered a new shifter (R3000) and a barrel adjuster. This is just conjecture so please excuse my lack of knowledge here, but my RD is an XT M772, which was always a bit of a bear to index, but with perseverance it worked well. I was told that a reason it was tricky to index was because of its increased spring tension. Could that stronger spring tension have somehow contributed to the failure of the shifter. its really not that old or abused. Just a wild guess.

If the new shifter works fine, here's an opportunity to attempt to learn about the old one. It will probably have one sticking ratchet pawl requiring disassembly to reach. If you can reach it, fix it, and against all odds reassemble it, then you'll have a working spare. You'll want a dental pick, and photos of every step of the way. If the project fails, no great loss and maybe some experience gained. Save some spare parts like the hood and plastic caps.

I disassembled a non-working 105 shifter once, and found a broken torsion spring on a pawl shaft. (Curiously, it worked when inverted.) It was unrepairable, but I later used the skill gained to fix another one.