Rear derailleur trouble shifting down.
 

Here's the deal:

Shimano 105 shifters and derailleurs. Bike is app 10 years old.

Several months ago the rear shifter handle started flopping when shifting down like there is no engagement at all. Shifts up (higher gears) fine.

So I figured the cable is hanging up, check my cables and found a little bit of corrosion at the top of the cable tube. Replaced the cable and housing worked for a while now its doing it again.

If I grab the cable where it goes down the tube and give it a little tug that kind of resets everything.

Before I remove cable from housing and rinse everything out, any ideas of other things I can look for?

When I put the bike in the stand it shifts fine this only happens "under load".

Another place to clean is inside the shifter. Spray inside with clean streak, work the shifter a few times, and use a compressor to spray out any dirt(lube and spray out again).

Not sure if it'll help, but it can't hurt.

When you replaced the cable, did you also replace the housing?

I have found that a "sticky" derailleur can also cause similar symptoms. You could try cleaning the derailleur and lubing all the places where the derailleur moves.

RD shifting troubleshooting is actually some of the easiest troubleshooting that can be done.

From your description I would have started with cables and housings. Make sure all of the pass through points are clean. It sounds like something is hanging up.

Make sure the derail is clean and nothing is causing it to stick. Use your hands to push it through the full sweep without any cable attached. (wheel out of frame as well).

Check the derail hanger. Loose? Straight? Use a DAG tool from Park to check it.

Check the wheel. Any movement in the axle with the wheel mounted in the frame (cone/bearing/locknut adjustment).

Check the cassette. There may not be play in the first few cogs but you may find movement from the middle cogs back. Comes from not getting the lockring seating properly/ having the first cog off.

All that checks out then pull the cable out of the shifter and route it like normal and shift it all by hand pulling on the cable. If it's all free and working correctly then you have a sticky shifter. If that's the case pull the shield on it and spray it with rubbing alcohol and blow it out with compressed air.

Done.

The derailleur has pivot points that can gum up. If you just replaced your shift cable and housing, I'd lube the derailleur itself.

Thanks for all the info, guys.

I've done all the things described.

I'm gonna double check the tension again. When its working it runs through all the gears fine.
If I up shift a couple gears sometimes that fixes it.

Gonna pull the cable and clean the cable and housings and lube the shifter.

im not sure if I understand you....when you click the shifter it just moves and doenst click and doesnt move the derailleur? if that is the case i just read yesterday that u need to clean the shifter out!!