Ghost shifting DT shifters

I have on my bike, what I perceive to be good quality shifters. Ultegra downtube 7 speed, but in friction mode to operate my 8 speed cassette.

I thought it worked perfectly, that is until I went somewhere with steeper hills. I found whenever trying to get up a hill and therefore putting more force through the drivetrain, the cable would pull the shifter back down, therefore downshifting from the biggest sprocket down to smallest ones.

The shifter itself seems in excellent condition, and shifts perfectly. Doesn't budge either unless trying to attack a hill.

What is the problem here?

Have you tried tightening the bolt on the side of the shifter that goes into the braze-on / band - is it a problem with the lever itself not being held tightly enough? The well known story is that this used to happen with older Campag shifters, which was why lots of the peloton used Simplex retrofrictions.

Yeah, sounds like they ain't tightened up enough..

If it wasn't tight enough it would shift down one or two, but not down to the smallest sprockets unless it was really loose...?

It's really loose.

Perhaps you need a spacer inside the shifter?

Its not loose at all, tight as it goes. Seems proper solid.

Time for some proper gears. 10/11 speed FTW.

fuck that shit. I would rather ride SS than have any more than a 8 speed cassette. Too expensive and redundant. Who needs that many gears? 5 speed would do me fine.

check your rear derailleur and its alignment across the rear cassette, both vert and horiz. also, is it a shimano cassette?
other than that, as above, the lever should stay put in all positions, and this could be where the limitations of using a device designed for 7 gears is becoming evident.

the cable would pull the shifter back down, therefore downshifting from the biggest sprocket down to smallest ones.

The shifter itself seems in excellent condition, and shifts perfectly. Doesn't budge either unless trying to attack a hill.

What is the problem here?

The cable must be going slack, it only pulls the mech to the larger cogs, it is the spring in the mech that pulls it onto the smaller cogs.

First try it on indexing, (7 speed will be fine on an 8 speed cassette), does this change the ghosting?

If none of the above helps and the screw on the shifter is as tight as can be then try cutting a washer from a bit of thin tupperware-esque plastic to slip inbetween the the shifter and washers that are already present.
I had the exact same problem and it solved it a treat as it allowed me to tighten the shifter a tiny bit more so it had no chance of slipping. Didn't affect the feel of the lever either.

Edit - EEI got there first.

Thanks for all the ideas, actually much more than I thought I would get!

Now to try them out...

My six speed Viner used to do this, turned out the shifter wasn't tight enough.

Did you solve this problem in the end? This happens a lot on my Vitus, but I think it might have something to do with the flexible frame... it's a large frame too so it can flex quite a bit. Will try some of these things out though

The cable must be going slack, it only pulls the mech to the larger cogs, it is the spring in the mech that pulls it onto the smaller cogs.

Just to make things clear, no.

The cable does not go slack, it gets TIGHTER and literally pulls the shifter back round. When on the largest sprockets the cable is obviously under the highest tension, and it is here where if I try to ride up a steep hill it downshifts.

Did you solve this problem in the end? This happens a lot on my Vitus, but I think it might have something to do with the flexible frame... it's a large frame too so it can flex quite a bit. Will try some of these things out though

I have not got round to it yet, going to try the washer thing. Not sure exactly where to place it though.

I need to buy new shifters entirely anyway to be honest. I think I might have left a piece out or something.... I'll let you know how I get on

Just to make things clear, no.

The cable does not go slack, it gets TIGHTER and literally pulls the shifter back round. When on the largest sprockets the cable is obviously under the highest tension, and it is here where if I try to ride up a steep hill it downshifts.
Could frame flex be the issue as this is happening on hills? Does the shifting only happen when you're stood up and putting the power down?