Please help with an intermittent problem

[djd's turbo]

I bought a 91 Spirit RT a couple of months ago. It has 150k miles, rebuilt engine, great shape. However, I'm having an intermittent problem that I need help with. A large part of the time it runs really well-no problems. But at other times it will 'skip' or 'hiccup' while I'm driving it. It seems to be worse after I have really pushed the car. My mechanic (who is good but not an expert on TIIIs) found a couple of vacuum leaks and fixed them. He cannot hear any more vacuum leaks. When this is happening it seems to idle high (around 1500 rpms). The rest of the time is idles around 900 rpms. The car has never 'died' on me when this is happening. I'm getting good gas mileage-around 28 mpg. Also, it is showing no codes. The car was not driven for probably 6 months before I bought it. Anyone have any ideas? Thanks in advance for any help you can give me. djd.

[djd's turbo]

Anyone have any ideas? Thanks.

[Boost_Virgin]

Check the vac lines and connections with wd-40. Spray some on the connectos, and if you have a leak you will hear the car change RPM. You may not hear a crack, but when you go into boost it opens up. Plus you can't always here a vac leak. Do you have a boost guage? What dose it read when the car is ideling on the high side?

[djd's turbo]

So you tend to think it is still a vac leak. OK, I'll try your idea with the WD40. No, I do not have a boost guage.
Anyone think it might have something to do with the fuel pump or clogged filter? I'm thinking maybe the pump is on it's way out or there is something that clogs the filter. Or, if it were that would it be doing it all the time?
Thanks for your help. djd.

[86Shelby]

A tried fuel pump or a dirty fuel filter will usually cause a misfire while under load and/or long cranking time to start.

[talanthalus]

Every time mine did this, the timing belt would break a few thousand miles later. Might have the belt tension checked, which is a good practice anyway. Do you hear a whining from the engine at all? These are notorious for breaking the timing belts if they're not properly tensioned, and since I went through a few, I got to know the signs pretty well. : /