The attitude of your doctor(s) is insane! Random fluctuations would be a reason to do MORE tests, not less.
Surely there must be some kind of patients association or other consumer organization in your country which defends people against technocratic abuses and profiteers. They might advise you on the best way to proceed in order to obtain rational care.
That said, you aren’t clear about your old test results (how much is above 300?). I can only assume you concluded from your old results that you could get by with 300mg.
In case you weren’t aware, getting flareups 14 months into allopurinol therapy wouldn’t be unheard of in a case such as yours. It doesn’t necessarily mean the 400mg dose was insufficient. You might have avoided that flareup by taking more allopurinol during those 14 months but increasing the dose after a flareup isn’t necessarily helpful. Blood tests would have helped to determine whether more allopurinol was needed.
Any allopurinol side effects could take a few weeks to go away.
And you should get a blood test done about a month after lowering your allopurinol dosage to make sure that hasn’t allowed your uric acid to reach an unsafe level.
If the dose reduction you devised doesn’t solve your problem and considering you have been gout-free for a while, you could also simply stop taking allopurninol for a few weeks and see if you feel better.
You will probably need some drug to keep your uric acid under control in the long run but it seems people who aren’t experiencing flareups can often take relatively short breaks without ill effects.
If you are indeed suffering from unacceptable allopurinol side effects, there are alternatives to that drug.
The most obvious one is febuxostat. It might help or it might not but it’s important to get blood tests done if you try it in order to prevent liver damage.