SRDiver

I just inherited a 1984 190e and other then showing it's age everything appears to work. However when you engage the cruise it surges / seems unable to modulate the trottle. The only time it is smooth is well it is under load (ie; slight incline) but goes all ***** once the road smooths out. There is no problem with brake lights etc. Is the problem the amp or the actuator?
Tim