Engine Performance problem
1997 Volkswagen Eurovan 4 cyl Front Wheel Drive Manual 200000 miles

Hello, when driving the van, between gear changing the revs stay up and return very slowly to idle, had many mechanics lok at it, and spent a fortune changing parts, can you give me an idea, parts changes, o2 sensor, idle control valve, water temp sensor, thorttle position sensor, plugs, leads, coil pack, ecu relay, no fault codes ever registered, now the dealer is saying i need to have the ecu checked/tested, far out? you help would be appreciated, Thanks George