98 Passat loosing power

[PJS1]

My manual 98 1.6 starts fine on a cold start and then after 5 - 10 mins cruising the revs decrease intermittantly with loss of power - feels like the engine is about to stall. When you clutch it the engine seems to recover and power loss may come back and may not.
On a warm start it takes a few seconds to turn over but starts eventually.
Seems like a general loss of engine power but intermittant.
Noticed the temp sensor on the dash was gone so cleaned the sensor and that seems to be OK now.
My mechanic thought it might be the temp sensor but a new one didn't help - he also cleaned the throttle body and it seems to have had some improved effect but the sluggishness is still there.
Doesn't accelerate well either - general low power feeling.
Plugs and Oil were changed recently with new air filter only 5K miles ago.
Haven't run diagnostics as it's very expensive!
Any suggestions on what this could be?