Newly acquired bike, chain issue?
Everyone on here seems extremely knowledgeable so hopefully you can help me out. I recently acquired a bicycle from my aunt that she had won in a raffle about three years ago. The bike was ridden for maybe 5 minutes after the win and then stored in a garage ever since. The bike is a Quasar Bounty (some cheap retail store brand I assume) and has a 3 gear setup (H and L settings for each gear set). Lately when I'm in the middle more commonly used gear setup (one in H, one in L) and try to stand up and pedal to get some momentum my bike makes a "pinging" sound and the pedals slip forward about a half rotation. Needless to say this kicks my feet off and causes me to lose all momentum on the hill. I've re-oiled the chain (assumed it couldn't hurt since it sat so long) but still no change. I'm hoping it was just a cheap assembly job and maybe something just needs adjusted. Please help me out, I'm itchin to ride.
3 year old bike that was never ridden has pedal slippage when pushed hard (up hills, standing up and pedaling). Trying to not die, help me out!
*And if pics are needed of any of the setup, let me know.