I have a '97 gmc seirra, 5.7 ltr. It has trouble shifting into 3rd gear and will not shift into 4th at all. When I say "has trouble" I mean I have to rev it up to around 5k rpms to make it shift and when im driving steady at 45 mph my rpms are around 3k-3500 rpms. I had a blown head gasket about 2.5 months ago and I replaced them. The truck sat for about a month with the blown gasket. I thought the fluid sitting for so long might have effected it. When I drained the trans fluid it had a slight burnt smel but not very strong and I added some "smooth shift" additive when I filled it. Fluid levels are good and temp of both eng and trans are fine but eating through fuel. Need help please.