bad ground OR more likely that the flipper coils have slightly melted and are sourcing too much current.

I am having this same problem on my TSPP. I checked the End of Stroke (EOS) switches and they are functioning perfectly. The lights dim in the back glass more noticeably than in the playfield with the right flipper button more than the left. Now there are 3 flippers on the right in TSPP and 2 on the left so possibly the increased dimming is related in that way.

My flipper coils look good and I see no melting though I'm not sure how to check them fully, what should I do?

How can I check for a bad ground?

@Flyer666, did you fix your problem?

I see a lot of people online just living with this problem but I'm afraid that will lead to something more serious breaking.

Excuse any errors in my math. This is my opinion on the problem:

98% of the time this will be solved by repinning all of the connectors (namely the power supply, using trifurcon female connector pins and new male pins, especially on every solenoid feed and GI feeds)
1% of the time the transformer is just too weak and old to supply enough current
1% entirely something else weird.

Well it's a machine built in 2003 so I doubt the transformer is too weak. I have been installing LED lights lately in the machine and while I heard that LEDs are less current overall I do worry that this has somehow caused it.

By repinning the connectors, do you mean unplugging and plugging them back in? I see the power supply board and I've done that already but I'm afraid what you're saying is more involved with the wiring.

Yes it is, for all the reasons why and great information check this link:
http://www.pinrepair.com/connect/