1999 Ford Ranger surges

Recently, if I'm driving at a steady speed (say 50mph) & I have to slow down for some reason, when I accelerate, my truck surges until I give it enough gas to make it stop surging. It also makes a "rattling" type noise. It seems to do it more when I'm going up a hill (even a small gradual incline). This just started about 2 weeks ago at the most. We changed the oil & filter, the fuel filter, and air filter (which had a mouse nest in it), to no avail! My husband & brother replaced either the entire muffler & part of the exhaust pipe ( or maybe just a section of pipe) a month or so ago. It also seems to lose speed up a hill even tho' I try to keep a steady push on the gas peddle. Any ideas - what next?
Thank you!