I also had a similar problem with a DE ST 25th...

Sometimes would have full power and sometimes would feel week..

Checked all the obvious.. coil sleeve, spring, resistance to the coil, it it was a mechanical or friction issue... all fine...

Pulled out the coil sleeve just to replace it to be sure and sure enough the coil had heated up at some point and expanded ever so slightly that i could not re fit the new sleeve. The old sleeve was fine as i could remove and refit with no problems..

The issue in my case was the coil....replaced the coil and have not had that problem since. Mind you the coil works fine and measure right its just that i can't get a new sleeve in it !

Just get a round file and carefully file out the inner plastic body of the coil.  Just make sure you don't go berserk and file through the plastic.  You should be able to file it down enough though to make it operational again and save some $$$$$.