I am far from the professional, but I would have to say that it has to do with the fact that you are burning your image for too long. 30-45 minutes even with a Halogen lamp I think is way to long. Before I purchased an exposure unit I used to use a 500w lamp, and would normally burn the image for about 13-15 minutes only and it would normally work pretty well. Good Luck.
My first exposure setup was with a 500w halogen lamp, and I burned the image for about 15 minutes at a distance of about 2.5 feet away.
I would agree with hawaiianphatboy that you are exposing the screen for too long. Try bringing down your exposure time until the entire image washes out of the screen cleanly.
As far as the emulsion, some have a shelf life of about a year, and begin to go bad after that. I would recommend getting some new emulsion if it is extremely old.
I regularly use a 500W halogen work light (with the UV glass removed). I can burn screens in 4.5 minutes -- perfect every time. 45 minutes is waaaaaaaaaay too long. See if you can track down a step test to perform to find the ideal exposure time. If you can't find one PM me and I can send you one that I have.
i lowered my exposure time to about 15-20 min and it worked out great for me!!!!
idk y but i thought that if i burned the image longer it will get an extra crispy burn and when i wash it out it will keep all its detail perfectly! especially if its a detailed image.
Light was eventually creeping in between your emulsion and your film and gradually exposing it all the way across. Buy some new emulsion anyway. I could gel up overnight on you. Six months is all it is designed to last once you put the activator in if a dualcure. If a one pot the life is even less. You need a fast one for that low wattage lamp. I use 5,000w.
Somebody already said it before, but if your emulsion is old, you'll run into problems. Check with suppliers, if it's been a long time since you've done screenprinting, they have samples of new emulsion out that takes alot less time to burn. The supplier I deal with in Az., gave me a quart for free to try out and see if it worked for me. It's murakami photocure blue. I have a 1k watt exposure unit and burn a screen in 1 min. They say it had a 6 month shelf life once you mix in the sensitizer, but I bought a gallon from them and it's been almost a year and I used the last bit of it and I still burned screens with no problem.
If you hold your positive up to a light, the more light that comes through, the harder the image will be to wash out. You want to use very opaque positives when exposing screens. Some inkjet inks are more opaque, adjust printer settings to lay down more ink, or double up the positives to give a darker positive. You are also exposing too long, and emulsion goes bad in heat.
I meant a very dark positive, meaning very opaque. As mentioned by Unik. If I double up my positives I can wash out my burned screens in seconds. The emulsion washes right out, as it should. If I don't double them up, it takes a bit more.
At a darkroom suply house you can purchase an opaque black pen for touching up film positives. They work great for small areas. Even a Sharpie will work on small areas. If the entire film ppositive looks grey when held up to the light then you need more ink down on the film to totally block out the light from coming through.
Burning image help!!! 
