My Eurovan loses power before reaching 1000 rpm, but it then recovers. It does not exceed 3000 rpm either. These failures appeared after an oil change. I work in an area of uphill and downhill roads.

When I bought the van, I clutched and it responded immediately, and it accelerated perfectly. However, it has now started failing. When cold, it does not run the same way as it did before and, after driving for an hour, I need to change the speed to 3000 rpm. On uphill roads, the vehicle loses power and, even though the engine strives, it is really slow. Please help me.