My first guess would be a slipping clutch. How many miles on the clutch?
You can test for slippage by accelerating (and/or popping the clutch) at a very low speed ( 10-15 mph) while in a higher gear (4th or 5th). If the engine revs freely the clutch is slipping. If the clutch is not slipping the engine should bog or at least struggle slowing against the higher gear.