Another thing to check, and I'm assuming its a 1" headset ?, but are they both the same standard either ISO or JIS. Google it and you will find the sizes. If the crown race is a loose fit on the forks that will exhibit movement.
Forget about the top nut for the minute. If you tighten the lower nut does the movement go ?. If not and the nut is tight then it sounds like you may be running out of thread on the fork steerer.
These are surplus so before they go in the box with all the other spare cranks I thought I would offer them up here. Used condition wirh scratches and scuffs so you might want to break out the polishing kit if you want some shiny cranks. Rings are 46/36/24. As you can see the middle is a...
Also it's the wrong stem for a 88,
it is the 89 style. Has a Deore II rear derailleur, one year only and that was 89 models. 21 speed shifters also indicates it's a 89 as 88 would be 18 speed. Also in 88 they didn't put the comp decal on the tt from what I recall but in 89 they did.
There was at least 2 colour schemes for the 89 Rockhopper comp despite what the catalogue may show. White and blue and yellow 100% I have them both but I think there was a black version as well, which I may even have but cant remember for sure so would need to check.
It's a 1989 model although it would have been available to buy in 88. The strange part you mention near the crank is a U brake. Note that it has no brakes mounted on the seatstays as you would normally expect to see.
Now let me start by saying I think this is a 94 Saracen Kili team frame. Im mainly going by the fact it had Deore LX components and Ritchey 440 rims. I guess it could be a Traverse FS but those components are wrong for that. Still happy to be corrected by anyone who knows for sure. Serial number...
Scott Pederson front brake here. Although read the description as it may not be a front, but probably is.
https://bicyclist.cc/product/scott-pedersen-se-cantilever-brake-black/