Multiple misfires 1,2and 3

[Walshie]

Hi. My car is a 1997 986 boxster 2.5. I have recently had the variocam solenoid on bank 1 replaced at my local Porsche main dealer. When I picked it up, I was told that I have a code for the pre cat oxygen sensor on bank 1 cylinders 1 to 3.have done probably less than 50 miles since I got it back and haven't changed the sensor yet. After a decent run, 60 miles or so, yesterday and after getting home, I got a CEL. After plugging in my obd2, diagnostic in I got the codes p0300, p0301, p0302 and p0303 multiple misfires, cylinders 1,2&3.
Is this because the sensor hasn't been changed yet or am I looking at an IMS problem.
The car has done 94440 miles but the IMS main seal was changed approx 6000 miles ago but is now out of warranty.
Many thanks