The particulars...stock 302, recently put all new seals/gaskets, replaced timing chain and oil pump and generally cleaned up. Was and is running really well, but has developed a strange performance characteristic. I have to cross the mountain to work each day, and if I try to accelerate to pass a slowpoke while going uphill, it seems to bog down...will not accelerate and sometimes sounds like small backfires. Occasionally I've experienced rough idle as well. It seems that the problem only occurs in third gear (C-4), but it also doesn't seem to be downshifting properly either at this point. It does seem to do it also if I try to accelerate on even ground above 70 MPH. That's about all I can think of at this point...any ideas? All input is appreciated.