Unusual pressures on semi-hermetic system

[Bunya nut]

I'm looking at an old split ducted system maybe 20kw or so. On start up, suction pressure will sit around 80psi (disch-230psi) for half an hour or so - with a superheat of around 12*k, and a full sight glass. Then something will happen to restrict flow sometimes back to 50psi-then it will go back to 80psi after 5 - 10 mins, but often it'll keep restricting back to 10psi (disch 171psi) with no cooling, frost on distributor and warm/ambient suction. If it gets that low, it doesn't seem to correct itself again until the compressor is restarted.
We have changed the tx valve and drier and didn't seem to make a difference. Txv is externally equalised. Compressor is a semi-hermetic type and has had it's unloader solenoid disconnected from when I first saw the unit before having this problem.

Any suggestions???:)

[Tesla]

Hi Bunya nut
I will assume R22 and at a guess I would say an intermittent TEV blockage possibly moisture? perhaps an oil acid test of gas test may show this. Or if you warmed the TEV with a bucket of warm water whilst at low back pressure then the pressure rises.

[monkey spanners]

What Tesla said, also is the evap nice and clean with pleanty of airflow and clean filters?

What happens to the suction superheat when the suction pressure drops?

[Bunya nut]

One of the guys went in yesterday to open the compressor up and put a new unloader and gaskets in etc... After that the system pressures are much better. Yes R22 - SP 65 - 70psi. 11k superheat.
Unit does have good airflow, coil and filters.
And for the record the superheat when the SP goes down to 10psi is very large (didn't get a value, but after the txv the distributer frosts up and the rest of the evap and suction line are warm/ambient to touch.
If it does start with this again, I would definately try warming the txv (thanks for the tip) will let you know if it continues thanks!
p.s. I thought an new filter/drier would pick up moisture if that was the prob though?

[Magoo]

Check the external TX valve, older ones have a habit of leaking internally particually if on R-22, which will shut the TX down or restrict flow. A tell tale sign is the external line sweats or even frosts.