Problems at speed
 

My 99 XJ8 with 81K miles I bought last month seems to be having some trouble that starts around 70MPH. It starts a low grumble that builds and slight loss of power. It doesn't start right away, usually takes about 15 minutes or so of driving at that speed. If I try to accelerate through it, it just gets worse. If I throw the gear selector over from D to 4 it gets much better, but still some vibration. No warning lights of any kind come up which is surprising to me because of how hard it starts to shake if you push it.
These issues are not present at slower speeds, it only happens on the interstate speeds

It seems to be ignition related (loss of one or two cylinders). I would first replace the spark plugs.

Thanks spark plugs were already replaced with NGKs. Would an ignition problem like a bad coil throw a code ?

Fuel starvation? Clogged fuel system?

I believe that a bad coil will, in most cases, throw a code but there may be cases, depending on how the coil fails, that it may not. I had a couple of cases with conventional ignition coils where, when cold, the coils would function well but, when they heated up after some time of driving, they would cause loss of power and have exactly the same symptom as what you described - the engine will run fairly well at a constant speed but it would not accelerate (it would tend to go down in rpm when you press the accelerator further). These were cases where the coils were still functioning but, when they heated-up, they would produce a somewhat weaker spark which cannot support acceleration. It is possible that coils producing weakened spark will not be recognised by the ECU.

However, your problem could also be due to fuel starvation (as mentioned by Eric) but this should normally not be heat related.

Hmm I did just replace the fuel filter, it appeared original and clogged up. I noticed the car idled so much better when replaced. I guess the next step is to measure the fuel pressure

Hmm OK do you have any suggestions on how to trouble shoot that besides just buying new coils?

I just measured a nice steady 41 PSI at the fuel rail with the vacuum hose removed from the regulator. So i dont think its a fuel problem.

Very hard to find out which coils are bad if they only start partially failing (reduced spark) when they heat up. This kind of problem is usually in their secondary windings (very thin wire) due to insulation deterioration. You can try measuring the secondary coil resistances on all coils and compare them; perhaps this will tell you something.

Since you probably have 4-wire coils, they have some electronics inside which could also be the reason for poor spark when heated up. Perhaps you can buy, say, 4 used coils and swap your coils one side first, test, then the other.

You can test COPs (using a COP tester) but commonly people buy one or more known good ones and proceed as just posted.

OK thanks guys, I ordered 4 for testing from here that is suppose to be decent used coils.

Update I finally got these in the mail from the UK ebay seller. All but one looked new. I dropped 4 of them on the drivers side since it was the easiest to get to and headed out on the interstate for a test drive. It is definitely better thanks guys! I could just barely feel the old shaking at speed and I drove all the way to the next town to make sure it was completely warmed up. I'm going to go ahead and order 4 more for the other side since they are cheap and mine are obliviously bad.

"Barely feel the old shaking" probably means that you now have just one more bad coil somewhere. Either one of the 4 you bought or one of the 4 in the other bank. I'm sure that, with more used coils, you will find it. Another option is, having determined where the problem is, to now just buy 8 new coils. Aftermarket ones are, at Barratt for example, GBP 28 a piece.

Yes, it's likely to show more than one, and you may see "restricted performance" message next to the check engine light.

If bad enough & occurring in near enough the same conditions, yes.
Otherwise, no.

Things can be wrong despite absence of codes, but when you do have codes something's wrong.