Ford F-250 4X4 Diesel 7.3L Emissions problem

My truck is a 2002 with 109,000 miles. The emissions light has been coming on periodically & then I notice either a hesitation or sometimes it actually dies, but will restart again. I have also noticed it happens when it has been raiining out. Eventially the light will go off and I won't have the hesitation or stalling. Do you have any good suggestions and just a "broad estimate" of how much it would cost to take it in to have it fixed? I am afraid of being ripped off. I have always had my own mechanic in the past until recently & I just would appreciate any suggestions you have for me. THANK YOU!