Suzuki Vitara Drops RPM on Highway

I have a 2002 Suzuki Vitara I have had for 5 years, and this was the first time I had any major problems with it. It was parked for about 3 hours, under direct sunlight, and when driving it home, once I hit the highway, I started having issues with the acceleration. The RPM was unstable and started to drop, and the gas pedal became practically useless. The fluid lights popped on, and I had to pull off the highway. Upon turning the car on and off, I could get about a mile before it would start again. I was able to pull off the highway on the next exit, and stop at a gas station. I checked the fluids, and the oil was low, and the coolant was empty (I rely on my father to maintain my car). I filled the coolant, waited a while, and started it back up. After revving the engine, and hitting high rpm, as well as allowing for it to idle, it seemed fine. Immediately I drove it to the shop 5 miles down the road. They did a check, refilled the oil, and everything seemed fine. Upon leaving, while I was driving city mph, it was completely normal. It wasn't until being on the highway again for about 15 minutes before the same issue began. Luckily, I was less than a mile from my exit, got off the highway, slowed my speed and all was fine.

I have no idea where to go from here. I have a 3 hour highway trip in a week, and need to get this checked out asap, but have no idea where to even start or what it could possibly be. Help!