Hi,
I had the exact same problem when I bought my golf. Got it cheap, guy had run it REALLY hot when matrix blew so I had to replace engine, complete rebuild anyway.
Got it all started, ran lovely and smooth, not a problem.
Took it for a test drive, GUTLESS doesnt discribe it!!! Got back quite annoyed cause of all the work, turned it off and had a look under the bonnet, nothing seemed out of place, tried starting it, it just turned over and over and over.
Tried a few things, crank sensor, new brain, temp sensors, no difference, took it to Vdub had a chat with a mechanic there and he threw it on the machine, G28 (crankshaft sensor) and camshaft sensor both came up, replace crank sensor again, no difference, replaced camshaft sensor and sorted the problem, hope this helps you.