I've recently (last 6 months) begun riding again after many years away and have begun updating components on my bike (a mid 80s era Fuji). After replacing the rear wheel and freewheel I developed a problem with the chain slipping. On a 7 speed freewheel, the 2 highest gears run fine, the 3rd gear slips somewhere between the 10th and 15th turn under load (but not when hand cranked with no load), the 4th gear slips so badly under load that it won't engage (again, I can't repeat this by hand cranking with no load), and gears 5 - 7 work fine. Thinking that it could be caused by the old chain I replaced it but the same conditions still occur.
All cables have been recently replaced. The rear derailleur was replaced about 1 month ago.
Any ideas would be greatly appreciated.