1998 Chevy Lumina Engine runs rough, loses power and stall

My car has run great up until recently. There was a smell of exhaust in the cab, oil leak, and antifreeze leak. At the suggestion of a local garage I replaced the valve cover gaskets, they told me that an engine of this age would have oil and antifreeze leaks so I was not as concerned about them. The gasket install went great, and the exhaust smell went away after I was done. About two weeks later my engine started to "skip" as I was driving. Almost like it wanted to stall but it would keep going. Everyone I talked to suggested the spark plug wires and/or the spark plugs. I replaced both, (followed instructions in a manual I bought at an auto parts store). No change, so I replaced the fuel filter. No change after that either. By now the engine started to stall when it was put into reverse after driving it for a while. I took it back in to the shop for another look. They reported to me that the injectors were working fine, my fuel system was ok (pressure was ok) and there were no errors shown when they connected the OBD computer. They were unable to feel the problem when they drove it. The moment I pulled out of the lot it did it.
The RPM's drop almost 1,000 every time it does it. Sometimes it will do it a few times in a row, regaining some power in-between "chugs". I took it into a third mechanic, he too was unable to get it to happen. Again I picked it up and it did it, so I turned around and picked him up. Luckily it did it a few times when he was in the car, and even stalled when I put it in reverse when I pulled up to the shop entrance. He said he wanted to try a few thing with it, so I left it with him. With the age of the car and the mileage I was not interested in spending lots of money to troubleshoot/fix the car. He said he would just try a few items. The ignition coils were not the issue, and some of the sensors (not telling me which ones) also had no affect.
He had no advice for me as to where to look or what to troubleshoot. I hope someone has an idea that won't cost a fortune. I like the car and would like to keep it for a little longer if possible.
Thanks all!