Possibly throttle position sensor

Hey there, so my fiancé and I bought an old beater (listed above) that ran and drove really smooth then after driving it for about forty five minutes after we got the registration and insurance we started seeing a really strange problem. So after driving it around the city trying to burn out some old gas we started seeing the car start to shut off. I had it quarter throttle and I saw the rpm's started to go down without me moving my foot on the gas pedal, so I brought it up to half throttle and it would fluctuate between one to two and a half rpm. Then I brought it up to full throttle, foot to the floor on the pedal and it goes between one rpm then up to three rpm and then back down to one rpm and it just keeps repeating that cycle. Also, when it started doing that we cannot get the car past thirty to forty km/h even with full throttle. We decided that maybe it was the fuel filter on the car. We took it off and found out the eighth of a tank of gas was really old and discolored, we blew through the old filter and it was clogged. So we got the new filter on, put some new gas and fuel stabilizer in the tank. Took it for a drive and it seemed nice again but after twenty minutes half way to our destination it started doing the same throttle problem again. My fiancé says he thinks it is the throttle position sensor but he has never seen anything like this before. So I am just looking to any other suggestions as to what it could be. My fiancé is a mechanic himself so he will be the one exploring everything and changing any parts that need replacing. And we cannot take it into his work (Chevrolet Dealership) because we are afraid that on the highway getting there he will have to put his hazard blinkers on and do forty k/m in an eighty km zone and that is very dangerous. Thanks for all your help guys! I am really at a loss here.