Boost comes and goes
2002 wagon, bone stock. 125K. Recent maint: fuel pump/filter, air filter - clean MAF, timing belt, PCV. When boost is working it works perfectly. Sometimes is fades to nothing when driving, sometimes it starts that way. Most times I can turn the car off, restart and boost will be there. I've checked: isolate wastegate - begins to open at 9 psi, full open by 15 psi, smooth operation. Vacuum lines OK. Intake hoses/connections OK. Clean and check stock boost controller - couldn't get to fail. If I unplug vac line to BC to bleed pressure, I never lose boost. Seems like the ECU is closing the boost controller - it doesn't act like a loose wire, and I haven't found anything mechanical. I've reset the ECU, no CEL's. Any ideas on what else I can check? Thanks.