Steve, the problem may be parasitic capacitance if the dummy load has multiple resistors due to the resistor lead end caps, the more resistors the worse the problem - check out QST article on UHF dummy load, it shows 11 x 560 ohm resistors in parallel and a small coil - looks like a very good article.
Regards
73 Paul 2E0YOB
Paul Simmonds
