2000 9-3 juddering / hesitant under load - DI?

[JayMax]

My car (2000 9-3 LPT) seems to be juddering and hesitant under medium acceleration, it's very noticable in 3rd and 4th gear at around 3500 rpm but clears up after 4000 rpm.
Feels a bit like it's choking or fuel starvation and then it runs fine. Has no issues at idle. Plugs were changed at the last service 4 months ago.

Is this likely to be a sign of the DI cassette being about to fail?

Thanks

[fiostech]

How many miles? When is the last time the O2 sensor been replaced, and spark plugs.

[JayMax]

She has done 148,000 miles - I think the previous owner had replaced the DI cassette at 100k but not sure unless I check the bills to confirm, and the spark plugs were replaced last service 4 months ago. I have no idea about the o2 sensor, unless that's a service replacement item I'd guess at never.