1995 Chevy S-10 Stumbling problem

Engine Performance problem
1995 Chevy S-10 6 cyl Four Wheel Drive Automatic 150K miles

The engine performs normally, but lately it's developed a sumbling problem where the rpm's will drop and there will be a smell like it's running too rich. After 20 or 30 seconds, the stumbling passes and the engine runs fine again. The check engine light never comes on, and there are not codes in the computer. There isn't a real pattern other than the engine is full warm when it happens. About 2 years ago the injection system was replaced, and so was the fuel pump. When the problem started, I replaced the throttle position sensor as well as running some injection cleaner through the fuel system. Before I start randomly replacing sensors, is there any particular sensor I should start with?