We had a booster installed which worked perfectly for around two years before it commenced to die and cause the signal to cut in and out. I ended up replacing it and didn't have a problem thereafter. It sounds like your booster is working [to some extent] so it may not be the cause but if you don't find a solution, it may be worth trying another booster in there.

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MY99 WRX (Sold)
MY06 WRX 241hp 430nm atw (Sold)
MY09 BMW 135i 247kw atw (Current)