Ghost shifting DT shifters

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?