2002 Rendezvous

I have a 2002 Buick Rendevous that is losing power. I have a very hard time getting up hills and when on the freeway, it almost feels as if I'm pushing on and off the break without actually letting go of the gas pedal. And sometimes on sideroads I can't get it to get past 20mph. My engine light is on, as well as my battery light, which flashes back and forth between battery and security. I'm not sure what this means, but I've noticed other electrical problems with car; the stereo doesn't always turn off even when the keys have been removed and occasionally the heat and/or lights won't turn on. It seems to shift fine, and doesn't feel like transmission, but obviously I don't really know. I purchased the car a year ago and took it on a roadtrip where I had a head gasket blow. I replaced the engine, but soon after the electrical problems started and now the loss of power. Any ideas on what's going on?