All last season and this year my car has always had a weird rpm fluxuation. If I hold it at 3500rpms it will drop 1 or 2 hundred rpms every 4-5 seconds and then jump back up to 3500. When it drops, it only drops for about 1/2 second or less. It will do this at any rpm starting point (not just 3500) The car seems to run fine and consistant, I just wanted to know what makes this happen. Its had 2 different carbs on it and a rebuilt distributor. So I dont think carb. or distributor is related to the problem. Thanks for any ideas.

spark scatter..
play in time ' chain and then dissy gear
once it get's to the rpm the parts"timing" move around and slow the rpm's ,till the slack is takin' up and it goes back to 3500 rpm.. and then it start the cycle again..
scope it..

Or one cylinder has an intermittent--but fairly regular--miss.

A 'scope test would find that, too.

NO, play in timing chain. The car is gear driven. Maybe, spark scatter but I have changed distributor cap, rotor and coil and plugs.

just cause you "changed it" (parts) doesn't mean if "fixed it"
scope it.. you'll see what cyl is missing or weak and or spark scatter...
dissy bushings loose...
dissy shaft end play????
pull on the shaft .
does it move up and down?
spring and weights not binding..
vac. advance working and no play in the linkage...
no holes in the daipham??(spelling)
does the vac advance ,hose have a little leak.. causing the avance to move around..