Stroker
What you have described is almost exactly what happened to me. About 3-4 years ago after finishing with a 199 average, I started to go downhill. It got to a point where I was lucky to shoot 400. I bought new balls, went to the local pro several times, did the video bit and attended several demo days. Nothing. My bowling just got worse. In the 2003/2004 season my average hit an all time low of 161.
In the fall of 2004, just after shooting one of my big 400 series and preparing to tie my bowling balls to the back of the car and drag them home for punishment, a friend of mine came over and said if I would stay after the league had finished, that he could correct my problems in about 15-20 minutes. I thought, yeah sure. After all the times with the local pro and many hours of practice, you can fix my problems in 15 or 20 minutes. This I gotta see.
The first thing he did was to put a towel about 2 feet out from the foul line and told me to throw the ball over the towel. That was not an easy feat for me as I usually put the ball down close to the foul line. But after a few tries, I was able to do it. Then he had me drop the ball down in my stance and make an effort to push it out along with the first step of my four step approach. Since I usually hold the ball about chest high, this felt funny. But after a few shots, it also began to feel natural. Next he had me stand close to the foul line and take only one step. This made me swing the ball and get the feel of what a free swinging arm would feel like. What he had me doing was just going back to basics. Put your hand in the ball, get it in a confortable position, look at your target, make a good approach and then just throw it off your hand. Don't think about anything else except making a good swing and hitting your target. As one of the old time pros used to say, "if youre thinking, your stinking".
Ok. The result. At that low point in my bowling and after some practice with this new advise, my average started to come back up. I went from a low of 153 in December of 2004 to a final average of 174. This year I am up to 187 and am bowling better nearly every week. ( If I would just stop missing some of the corner pins) My timing has come back, I can get the ball out on the lane better and I have a lot more confidence. For the past 5 weeks, my average has been over well over 200.
So, what I am trying to say, is try to get back to the basics. Forget about everything but making a slow approach, looking at your target and "throw the ball off your hand". If it is drilled right, and your hand is in the correct position, that's about all you have to do to make good shots. Oh, by the way, you should also remember this is just a game and try to have fun. Yeah, sure. I only have fun when I bowl good and beat everybody. Another perk for me, is that once I started bowling better, I did not feel the need for as many new balls. ( This really made my wife happy, however, didn't help with my relationship with the proshop )
By the way, I am using a wrist support and it helps an old man like me get a little more revs. on the ball. It also, helps me get the thumb out faster. Its not the one with the extended index finger, although that might help moving your track up above the thumb hole. I may try one of those in the future.
Sorry this is such a long post, but I hope it helps a little.