Strange thing happening to one of my older Macbooks, and have tried just about everything. It will charge just fine if I leave it alone, but charger turns on and off repetitively as soon as I start using the Mac while it's plugged in. Tried installing a new charging port and did not solve the problem, has a new battery too that is third party. Did reset SMC, and tried different chargers but no luck. It's a 17" from all the way back in 2010, but has a Quad Core i7 with 16GB of ram so would love to keep using it.