This can be done digitally on the cheap and can be very stable. Two options off the top of my head are (1) A microcontroller either with a D-A output or interfaced to one, (2) an EPROM (or EEPROM or OTPROM, whatever is cheapest) interfaced to an D-A converter and programmed with a sine pattern. Its address bus could be driven by a counter IC with  timing from a 555-type timer.  The microcontroller approach is probably the cheapest and lowest part count option.

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