1985 Trek 500, 1986 Trek 500 Tri Series, 2002 Orbea Team Euskaltel, 2005 Cannondale R1000
try this - flip your bike over in a nice quiet place and spin each pedal individually; listen and feel for vibration, noise, etc.
try removing your chain (You can just pull it off the chainring and rest it on the Bottom bracket sheel of the frame) so that you can turn the cranks without the chain. Again listen and feel for the binding or grinding.
Hopefully this will isolate where the source of your problem is.
An old mechanic's trick is to put a screwdriver blade against the part you are listening to and putting your ear up against the handle to amplify the sounds, but this will be hard to do with turning parts, and you prolly won't need to anyway.
If there is no discrernible noise/grind, try turning the cranks again, but with the chain on - maybe it's the chain that's toast or dried out.
a good overhaul with some fresh grease never hurts, so if it comes to that check out a repair book in the local library for pics and tips or use a good local LBS if you don't want to diy. If your BB is a sealed cartridge it's prolly cheaper to replace.