I'll tell you where to look, underground!
I did a service call for a Bell TV customer three or four years ago, experiencing very similar problems.
He also had around 150 - 200 feet of cable underground.
Adjusting the dish, changing the LNB's did not change things.
So, we dug up the line and guess what we found?
The Bell Tech who installed it must've run out of cable and attached a new piece using the proper F81 barrel connector WITHOUT any weather protection.
When I opened up the connections, they were corroded.
I replaced the entire length of cable, he got all his signal strength back, and I haven't heard from him since.
So, did you install & bury the line? If you did, can you remember if the line is a single piece?
Maybe there is a small knick or cut somewhere along the buried portion of the line, and when it rains, water is absorbing into the ground and getting into the damaged section.
Also, I believe 200 feet of cable does not require any kind of in-line amplifier.
I'd ditch that too, in my opinion.
Honestly, if I was you, I'd replace that entire line, be sure you use the proper RG6 coax, and make it a single piece.
The time required will less than all the troubleshooting.
I just thought of another idea; it could be a receiver problem. If you know someone with a working Shaw receiver, ask them to try it out at your place, and install yours at their place. If you have a cable/dish problem, their receiver will act up too.