If it runs fine once the engine gets above 3500 or so RPM, you might need bigger pilot jets. If it runs fine below that speed, the main jets could be too small.
Could also be that the slide springs are getting a little soft and the slides are opening too soon (or not closing up when you reduce the vacuum by opening the throttle). This causes a drop in air flow and the mixture leans out because there isn't enough pull on the feed jets.
