I'm not suggesting that this is your problem, but I can't relate your symptoms to anything else I have ever experienced other than common tuneup issues.

To rule out the clutch assembly...

Get in fifth gear at the lowest possible RPM the car will run at. Probably somewhere around 40ish. Then as your moving along, floor it. If the RPM's go up and your speed does not, your clutch is slipping. You'll feel it.