I am new to the GMTruckclub forum, and forums in general. I hope to gain insight from other GM/Chevy owners, and hopefully offer some as well! In July, I bought a 1997 GMC Suburban SLE with 152K miles. Love this truck. It was well cared for by the previous owner, but in need of some routine maintenance. I've spent nearly $2000 on updates and repairs--everything from tires to a water pump. Recently, I took it up to Vail--about a two hour drive from Denver. It performed perfectly going up but on my trip home, I could not get it to go above 45-50mph going up Vail pass, nor up to the Eisenhower tunnel. At times, it felt like a tranny issue. I'd accelerate and the RPMS went up to 3-4000 but the vehicle did not respond. Other times, it felt like it was starved for fuel. I floored it and there was nothing--no revving, no response from the vehicle. The 'service engine light' came on but as soon as I got past the tunnel and to lower elevations, it was fine. I should note, it "ran" fine the whole time too... there was no puttering, stalling, etc. Since then, the service light has gone off by itself and driving around town in Denver, I cannot get it to repeat the issue. A technician at Advanced Auto Parts as well as my mechanic ran a diagnostic and O2 senors (1 & 2) were the culprit. Problem is, both of these have been replaced since I bought the vehicle in July. My mechanic told me the codes showed that it was running lean, making him wonder if it was a fuel pump issue. Coincidently, it was running too rich back in July, which prevented it from passing emissions, which is when one of the O2 sensors was replaced to make it pass. Curious if anyone out there has experienced similar problems and has advice. I trust my mechanic but I cannot help but thing this has something to do with altitude given the locations where it started acting up.