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A trumpet is made of metal and any changes by only playing it take a fairly long time! The chance of a horn getting "more and more out of tune" without any obvious mechanical damage or leaky corks is very close to zero. These are all things that a specialist checks first!
I have played many older badly taken care of Bachs and even with valves out of line and an unbelievable amount of grunge in the horn, less stuffiness is the only major improvement after cleaning - not any serious intonation issues.
There is no "common problem pitch" on a Bach 37.
So what to do?
Check the spitvalve to see if it seals (cover the hole with your finger and play - if the problem goes away, you have found the most common problem)! As you have changed tuning slides this is probably unlikely though.
The next thing you will need to have checked by a pro - the valve alignment.
Solder blobs or bad solder joints do not change with time, the problems that they cause would be pretty constant!
If your breathing is not together, the 37 could just be a tough blow for you - that will cause screwy intonation as fast as anything mechanical!!

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Whenever I feel blue, I start breathing again.