Boxster Misfire - suggestions?

[Legba]

Can anyone advise on the following please?

Boxster 2.5 1998 63000 miles.
Serviced 800 miles ago including new spark plugs and air filter. It had been misfiring prior to this and it improved it but it is now worse than ever.
The car is not used often and when it is used it is usually for short runs. However, I did take it for a long run last week and since then the problem has been worse.
I have not identified any air leaks.
I have tried disconnecting the MAF and it made no difference.
I have cleaned the MAF with electrical contract cleaner. Prior to cleaning it looked a bit dirty and is now clean. This made no difference.
As this problem is getting worse and the weather is getting colder could there be a connection?

Symptoms:
Cold start idle is fine. Hot idle is fine.
On revving above about 2000 rpm there is a misfire and revs drop to idle speed.
If I run it and rev it long enough for the engine to reach operating temperature the engine runs ok.
For some time prior to this the engine occasionally misfired about 4000rpm but this would eventually clear.
I borrowed a fault code reader and it showed misfire on 123, I cleared it and ran the car again and this time it showed misfire on 456. No other faults shown.

[mikefocke]

Change the AOS ever on the car? Oil in the intake tube? Any smoke?

Ever changed the 14 year old coil packs?

Worst kind of driving you can do to the car is infrequent short runs.

[Legba]

I have read of AOS problems so that might be a possibility, and I have never charged the coils so thanks for those suggestions.

It does smoke a bit on start up occasionally but there is no apparent oil use and I thought this was just one of the Boxster characteristics.

The car was MOTd a couple of weeks ago and emissions were fine.

I warmed the car up this morning and it seemed to be running well so I took it for a fast run. It cruised smoothly at 85 for a couple of miles then started to misfire and lose power. OBD showed misfire on 456 so my thought that this was only a cold running problem was wrong, although it is much worse when the engine is cold.

Thanks for the suggestions so far, any other suggestions are very welcome.

[The Radium King]

i agree with coil packs as an issue. it is odd, however, that it is swinging from one bank to the other. bad spark could be due to fouling of plugs, but then one would expect sign of oil in exhaust. perhaps also bad spark/misfire due to leaking injectors, but codes should indicate mix problems. similarly, symptoms also indicate potential fuel delivery problems (plugged filter, bad pump or pressure regulator) but again codes should indicate mix problems.

[Legba]

Thanks for the suggestions, I have removed and checked the plugs and there is no sign of fouling and there are no other fault codes.

I think I will replace the coils.

Can anyone suggest anything else before I spent lots of money?

[The Radium King]

sniff your oil; if it smells like gas your injectors might be leaking. a cheap test, but still sounds like coils.