wet willie

Greetings and salutations! Oh and Happy Holidays as well. I just joined the forum with the hope of getting some diagnostic advice on my TH400. Its shifting performance is pathetic. Specifically, it shifts at 2600-2700 RPM regardless of the load. I'm of the opinion that this means that either the modulator is bad or I there's a major vacuum leak but I'd like another opinion. The tranny doesn't slip at all. Its just that it waits until 2600-2700 to shift even under no real load. I just had the filter cleaned and fluid (which was burnt) replaced. I'd sure appreciate some advice on how to repair it and get the shifting action back to where it should be. Thanks alot!