Why is the light switch for my foyer tripping the GFI in the laundry?

I replaced the plug over the laundry sink with a CGFI. I've done this many times so I know it's not a problem with my installation there. It is the first receptacle on the circuit that feeds the foyer, bedroom hall, laundry hall and laundry, mud room, and garage lights, so when this pops, I lose a lot of... show more I replaced the plug over the laundry sink with a CGFI. I've done this many times so I know it's not a problem with my installation there. It is the first receptacle on the circuit that feeds the foyer, bedroom hall, laundry hall and laundry, mud room, and garage lights, so when this pops, I lose a lot of lights. The foyer has three switches on three different walls. when i hit any of the three foyer switches, it pops the CGFI in the laundry. When I use any of the other switches for the other fixtures on the circuit, they work fine and nothing pops, so it's only the foyer. The first CGFI would not reset but then I remembered I had a problem with that CGFI last year when I installed it in the kitchen, and removed it. I installed a different CGFI and it still pops but I can reset it. So obviously it's a grounding issue but is it with the switches or the fixtures on the switches (there are 2 fixtures in the foyer). The switches were upgraded two years ago (by someone else) so do I need to go into those switches and check for grounding wires? Is there anything else it could be? oh, and the circuit is 15 amp and I used a 20 Amp CGFI (it's what I had in the house).
9 answers 9