Engine computer woes

I have owned a 1999 Mercury Sable (109,000 mi.) For 7 years. Bought used. Ford Program car. From the beginning the wipers would activate themselves with turning on the left blinker, and at times for no known reason. "Computer glitch, live with it if that's all it is, " was service advice when I asked dealer shops.

Last summer in Arkansas hot weather (about 100.000 mi. Then), as I would travel along steady between 55-60 mph, the tachometer would very briefly race upward about 1,500-2,000 rpms. It gave the impression of kicking into passing gear, but the transmission seemed uninvolved, only the engine. It might do this two of three times within ten seconds. As I slowed down to 50mph, then I could accelerate back and things might be okay until another very steady session in that speed range could bring it on again.

Now, it has done the same tach thing at other speeds, once idling at a traffic light.

The car is perfectly good otherwise. Do I need a new computer? If so, are aftermarket computers available?

Long question, sorry.

Thanks.

Richard Akins