This worked with my Lincoln 120Amp MIG welder, however, different models may have different designs, so take it slow, inspect everything, and think before you act.
Tools needed: none
Step 1: Safety
Step 2: Open Up the Handle
I recommend against using pliers since the parts are all plastic and easy to crush. If you need extra help I'd try wrapping the stubborn part in paper towel before gently using a set of channel lock pliers on it. DO NOT use any oil to loosen the parts. The switch mechanism relies on clean metal to metal contact, which most lubricants will prevent.
Once the collars are off the handle should separate easily into two halves. When you pull the halves apart the trigger will fall right out. It is spring loaded, so It may pop away from the handle a bit, so be ready for it.
Step 3: Inspect the Inside of Your Gun
The bare contacts are normally pressed into little plastic slots which keep them aligned for the copper ring. If you look closely at the picture of the contacts, you will see that the top contact is not seated properly, and the contact tip is pushed down into the gun body. This was keeping that contact just on the edge of the copper trigger ring. This meant that, depending on how I held the gun, the contact might be made or stutter on and off. This little mis-alignment was the cause of all my problems. I suspect that the contact came loose when I accidentally banged the gun against something, and just kept working itself looser over time.
I corrected the problem by simply pushing the contact back into it's seat. You could probably add some glue if you wanted, but this fix was very easy, so I'm going to see if the problem happens again before taking that step.
Step 4: Re-seat the Trigger
Everything should go back together easily. If it feels like you need to apply any force there is something wrong, and you need to go back and check that everything is lined up properly. It can be a little tricky to hold the trigger in while you position the top half of the gun body, but mine only took a couple of tries before everything lined up nicely.
Then just keep the two halves held firmly together with one hand and screw the upper and lower collars back on with the other hand
Step 5: Finished
Good luck and happy welding.