Laptop shuts down when moved, difficult to turn back on

Hello everyone, 

I've been struggling to fix my Dell Inspiron 15 (7568) 2-1 now for a couple weeks. I'd like to fix it myself if possible, but if not I need to decide if I should send it in (no warranty - 4 years old) or give up on it and get a new one. It hasn't experienced any problems until now so I hate to give up on it. Summary at bottom.

Full Story:

The original crash happened after using the laptop until the low battery alert came on. I plugged it in and walked away. When I came back it appeared to have gone to sleep, but wouldn't come back on. When I plug and unplug the power adapter the light indicating it was charging does turn on and shows it's charging. After leaving it overnight (with this light indicating it was charging) the light no longer stayed on when plugged and unplugged, indicating that the battery was likely fully charged. Still wouldn't turn on. Tried all power scenarios (plugged in with battery, plugged in no battery, battery no plug). Nothing worked.

Took the laptop to a local shop. They tested the power adapter and said it was fully functional but may be a DC power jack issue. I was skeptical because I felt it was charging, but I ordered a new part from Dell and installed it (no soldering required for my model). To install I did remove and reseat the battery. AND IT WORKED! Until I shifted it in my lap by picking it up on the lower right hand corner as I moved and INSTANT full shut down. Before that it worked just as it had before. I realized that I probably did the same thing when I plugged it in before. I feel like this is key. 

Decided to start troubleshooting myself. I redid the instillation of the DC power jack and it didn't come back on. Took off the back, blew out the dust, tightened screws (retest-nothing), cleaned the fan and unplugged and replugged it, checked other connections. Everything looks fine to my untrained eye. Decided to test it before screwing the base cover back on (essentially lifted it and held it upside down, keyboard facing down) and it came back on! put on back and gently flipped it and it stayed on. 

Now I had a chance to run internal diagnostics. Ran Dell 40 min diagnostics test and got an all clear. No issues detected. Battery showed charged to 50% when it turned on. I left it on the table overnight, checking in on it and using it without any problems the next day. Functioning just as before. 

Decided I needed to back it up to the external drive before it crashed again so I (gently) picked it up to bring in into another room. Does fine. Then I (apparently not as gently) moved it from one table to another where I once again held it with one hand momentarily. Lowered it to the table and it crashed.  

I managed to get it to come back on once upside down like before, but when I flipped it over it crashed. I have also reseated the RAM. 

At this point I'm thinking the soldering must be cracked somewhere on the motherboard causing it to force shutdown when picked up, flexed, or flipped. I have been unable to get it to come back on since then. Since it lasted all night and over an hour while working, and crashed specifically when moved, I don't feel it's an overheating problem, but could be a fail-safe around these systems. 

Any suggestions? 

If it is a bad solder joint, any resources or advice to find it and re-solder? (I have full access to electronic soldering equipment (including flux and solder) Just don't have a ton of hands on experience. Should I just send it in?

Summary: Laptop crashes when picked up. Comes back on intermittently when inverted but generally difficult to get working again. Adapter has been checked and DC Power Jack replaced. 40 min Dell diagnostics show no issues. No clearly broken part (untrained eye). Many parts have been reseated including battery, DC Power Jack, fan plug, RAM, etc.