Jim;

If #2 and 3 plugs are fouled and those cylinders are not contributing to making power, clean them, and get those cylinders working first...but I agree with Ozzi. My first idea also, would be frozen centrifugal advance. Info on this at: http://www.intelab.com/swem/Volvo%20Ignition%20from%20Scratch.htm#Detailed%20CA%20Servicing

...and remeber, an engine at idle is under practically no load, and so many times will run just fine, but as soon as you try to get it to do some work, you'll notice that something is not right...blipping the throttle is a quick way to see it the engine is capable of putting out power...by blipping the throttle, the engine spins up against its own inertia..so it should be able to do that first...if that seems OK, its time for a road test.

Good Luck, please let us know what you find.